UC-NRLF 


UNIVERSITY  FARM 


A    SHORT    COURSE    IN    THE 

TESTING  OF 

ELECTRICAL  MACHINERY 

FOR   NON-ELECTRICAL   STUDENTS 


BY 

J.  H.  MORECROFT,  E.E.,  B.S. 

FELLOW  A.I.E.E. 
Associate  Professor  of  Electrical  Engineering,  Columbia  University 

AND 

F.  W.   HEHRE,  E.E. 

MEMBER  A.I.E.E. 
Assistant  Professor  of  Electrical  Engineering,  Columbia  University 


111   ILLUSTRATIONS 


FOURTH  EDITION,  REVISED  AND  ENLARGED 


NEW  YORK 

D.   VAN    NOSTRAND   COMPANY 

EIGHT  WARREN  STREET 

1921 


COPYRIGHT,  1911,  BY 
D.  VAN   NOSTRAND   COMPANY 


COPYRIGHT,   1915,  BY 

D.  VAN  NOSTRAND    COMPANY 


COYPRIGHT,   1921,  BY 

D.  VAN  NOSTRAND  COMPANY 


PREFACE  TO  FOURTH  EDITION 


SINCE  bringing  out  the  last  edition  of  this  text,  the  engineer- 
ing courses  at  Columbia  have  been  put  on  a  graduate  basis; 
the  amount  of  electrical  work  in  the  courses  for  non-electrical 
students  has  been  materially  increased,  making  it  possible  to 
carry  out  the  laboratory  work  along  broader  lines  than  was 
formerly  done.  We  have  added  certain  experiments  on  batteries, 
illumination,  measurement  of  electrical  energy,  etc.,  with  the 
idea  that  the  non-electrical  engineer  frequently  has  to  pass 
judgment  on  these  phases  of  electrical  installations  and  should 
therefore  have  a  passing  knowledge  of  such  features  of  electrical 
engineering. 

We  are  glad  to  acknowledge  indebtedness  to  our  colleagues 
Prof.  M.  Arendt  and  Mr.  W.  A.  Curry,  who  have  given  ma- 
terial assistance  in  the  preparation  of  this  edition. 

J.   H.   M. 

August  i,  1921.  F.  W.  H. 

in 


PREFACE    TO    FIRST   EDITION 


IN  presenting  these  brief  notes  the  authors  feel  that  an 
explanation  of  their  object  is  necessary.  At  Columbia  University 
practically  all  of  the  engineering  students  are  required  to  take 
courses  in  the  electrical  laboratories,  testing  both  direct-cur- 
rent and  alternating-current  machinery.  Students  in  Mining, 
Mechanical,  Metallurgical,  Chemical,  Civil  Engineering,  etc., 
do  not  have  those  courses  in  the  theory  of  electrical  machinery, 
which  are  really  necessary  for  a  proper  comprehension  of  the 
machines  with  which  they  work  in  the  laboratory;  it  is  unrea- 
sonable to  expect  them  to  consult  various  text-books  to  prepare 
themselves  on  the  theory  involved  in  the  tests,  and  it  is  with 
the  intention  of  filling  the  needs  of  these  men  that  the  notes 
have  been  compiled. 

Before  giving  specific  directions  regarding  the  test  to  -be 
performed,  a  brief  analysis  of  the  characteristics  of  the  machine 
is  attempted;  in  so  far  as  is  possible  in  such  a  limited  space  the 
reasons  for  the  behavior  of  the  machine  are  given.  It  is,  of 
course,  realized  that  a  complete  analysis  of  the  different  types 
of  machines  is  impossible  and  it  is  questionable  whether  a  com- 
plete analysis  would  serve  the  purpose.  It  has  been  the  inten- 
tion of  the  writers  to  present  the  subject-matter  in  such  a  manner 
that  the  student  not  well  versed  in  electrical  theory  can  get 
the  most  out  of  it  in  the  short  time  allotted  to  the  electrical 
courses. 

In  some  of  the  tests,  methods  are  described  which  may  not 
be  strictly  according  to  the  standard  practice;  if  a  gain  in  sim- 
plicity and  ease  of  performance  is  to  be  obtained  by  a  sacrifice 
in  accuracy  of  the  test  of  a  few  tenths  of  a  per  cent,  it  is  thought 
justifiable  to  use  the  simpler  method  of  testing. 


vj  PREFACE 

While  the  notes  are  being  put  into  printed  form  specifically 
for  our  use,  they  may  possibly  be  found  of  use  in  other  schools 
where  the  conditions  are  similar  to  those  at  Columbia. 

The  authors  wish  to  express  their  indebtedness  to  Professor 
Geo.  F.  Sever,  who  first  developed  the  electrical  laboratory  work 
for  the  non-electrical  students  at  Columbia,  and  whose  original 
schedule  of  experiments  served  as  a  guide  in  arranging  this 
work;  also  to  Mr.  F.  L.  Mason,  who  has  rendered  valuable 
assistance  in  the  preparation  of  the  book. 

J.  H.  M. 
F.  W.  H. 

COLUMBIA  UNIVERSITY, 
September,  1911. 


PREFACE   TO   THIRD   EDITION 


IN  bringing  out  the  third  edition  of  this  manual  we  have 
thought  it  well  to  expand  some  of  the  experiments  and  to  add 
one  on  the  location  of  faults  in  direct-current  generators  and 
motors.  We  have  also  added  many  questions  which,  it  is  ex- 
pected, the  student  will  answer  in  writing  his  report.  The  ques- 
tions have  been  so  selected  as  to  show  the  main  ideas  which 
should  have  been  gained  from  the  work  in  the  laboratory. 

J.  H.  M. 

F.  W.  H. 

February  i,  1915. 


LIST  OF  B.C.  EXPERIMENTS 


1.  "FALL  OF  POTENTIAL"  ALONG  A  CONDUCTOR  CARRYING  CURRENT. 

2.  MEASUREMENT  or  ARMATURE  CIRCUIT  AND  SHUNT  FIELD  RESIST- 

ANCES. 

3.  THE  SHUNT  GENERATOR;    PRELIMINARY  WORK  WITH  A  GENE- 

RATOR.   MAGNETIZATION  CURVE;  EXTERNAL  CHARACTERISTIC. 

4.  THE  COMPOUND  GENERATOR;   ARMATURE  CHARACTERISTIC  OF  A 

SHUNT  GENERATOR;  EXTERNAL  CHARACTERISTIC  OF  A  COM- 
POUND GENERATOR;  EFFECT  OF  OPERATING  A  COMPOUND 
GENERATOR  AT  SPEEDS  HIGHER  OR  LOWER  THAN  RATED  VALUE. 

5.  THE   SHUNT   MOTOR;    SPEED   CHARACTERISTICS;    COMMERCIAL 

EFFICIENCY  BY  BRAKE  TEST. 

6.  THE  MOTOR  STARTING  RHEOSTAT. 

7.  EFFICIENCY  OF  A  SHUNT  MOTOR  BY  THE  STRAY  POWER  METHOD. 

8.  THE  SERIES  MOTOR. 

9.  CURRENT-TORQUE  CURVES  OF  DIFFERENT  TYPES  OF  MOTORS. 

10.  PARALLEL  OPERATION  OF  SHUNT  GENERATORS. 

11.  PARALLEL  OPERATION  OF  COMPOUND  GENERATORS. 

12.  THE  COMMUTATING  POLE  MOTOR  AND  GENERATOR. 

13.  LOCATION  OF  FAULTS  IN  A  DIRECT  CURRENT  MOTOR  OR  GENE- 

RATOR. 

14.  THE  DIRECT  CURRENT  WATT-HOUR  METER. 

15.  THE  LEAD  STORAGE  BATTERY. 

16.  ILLUMINATION  LAWS  AND  MEASUREMENTS. 

vii 


TESTING  OF  ELECTRICAL  MACHINERY 

DIRECT  CURRENT  TESTS 


EXPERIMENT  I 

"  Fall  of  Potential  "  along  a  Conductor  Carrying  Current. 

(a)  If  an  electromotive  force  is  impressed  upon  a  circuit,  as  for 
instance  a  wire,  a  current  of  electricity  will  flow  along  it.  The 
relation  between  the  current,  resistance  and  difference  of  potential 
between  any  two  points  on  the  conductor  is  given  by  Ohm's  law, 
which  expresses  the  equality  of  the  impressed  force  and  the  reacting 
force.  It  is  found  that  the  reacting  force  varies  directly  with  the 
current  and  this  fact  may  be  expressed  by  the  equation, 

E=IR 
from  which  we  obtain, 

I  =  E/R,     ........     (i) 

where  /=  current  flowing  in  amperes; 

E=  difference  of  potential  in  volts,  between  the  two  points 

considered; 
R=  resistance  of  the  conductor  in  ohms,  between  the  two 

points  considered. 

It  is  a  fact  that  all  conductors  offer  more  or  less  resistance  to 
the  flow  of  an  electric  current  and  experiment  shows  that  for  any 
particular  conductor,  the  resistance  varies  directly  as  its  length  and 
inversely  as  its  area  of  cross-section.  This  maybe  expressed  in 
the  form  of  an  equation  as  follows: 


(2) 


2  TESTING    OF   ELECTRICAL    MACHINERY 

where  R  =  resistance  of  conductor  in  ohms; 
/  =  length  of  conductor  in  feet; 
a  =  area  in  circular  mils; 

K=  resistance  per  mil  foot  of  the  material  used;  i.e.,  the 
resistance  of  a  conductor  one  foot  long  and  having  a 
cross-sectional  area  of  one  circular  mil. 

If  the  value  of  R  as  given  in  Eq.   (2)  is  substituted  in   Eq. 
(i)  we  have 


^ 

K.  — 

a 

and 

E  =  IK-.  (4) 

a 

This  states  that  when  the  other  factors  remain  constant,  the 
"  drop  "  in  potential  varies  directly  with  the  current  and  the 
length  of  the  conductor  and  inversely  with  the  area  of  cross- 
section.  The  drop  also  varies  with  the  material. 

It  is  also  evident  if  Eq.  (i)  is  written  in  the  form 


(5) 


that  the  resistance  of  a  conductor  is  readily  determined  if  the 
current  flowing  through  it  and  the  drop  in  potential  across  it 
are  known. 

The  apparatus  to  be  used  in  verifying  the  laws  stated  above 
should  consist  of  a  board,  upon  which  are  mounted  and  con- 
nected in  series,  equal  lengths  of  wire  (preferably  48  inches) 
of  copper,  aluminum,  iron  and  German  silver,  all  of  equal  cross- 
section.  Connect  the  wire  board  as  shown  in  Fig.  i.  Determine 
the  safe  current-carrying  capacity  of  the  German  silver  wire 
and  be  sure  that  the  variable  resistance,  lamp  board,  and  ammeter 
can  safely  carry  this  current.  The  voltmeter  should  have  a 
capacity  on  one  range  of  about  15  volts  with  another  range 
lower  than  this. 


"  FALL   OF  POTENTIAL  "  3 

With  only  one  lamp  in  the  lamp  board  and  all  the  resistance 
in  the  variable  rheostat  cut  out,  close  the  switch  and  note  if  the 
ammeter  deflects  in  the  proper  way.  If  not,  open  the  switch 
and  reverse  the  ammeter  connections.  Then  cut  in  enough 
additional  lamps  in  the  board,  so  that  the  current  will  be  slightly 
greater  than  -J-  of  the  safe  current  value  for  the  German  silver 
wire.  Insert  enough  resistance  in  the  variable  rheostat  so  that 
exactly  \  of  the  maximum  permissible  current  is  flowing.  Using 
the  highest  range  upon  the  voltmeter,  place  one  lead  to  one  end 
of  the  copper  wire  and  tap  the  other  voltmeter  lead  on  the  6-inch 
mark  upon  the  wire.  If  the  throw  of  the  needle  is  in  the  proper 
direction,  the  free  lead  may  be  fastened  to  the  wire;  if  not,  the 


f 

-vr- 

-<3r 
-o- 

-o- 

-o- 

f            f                                                                 Lamp 

101 

t     A     1                /VAA/\A.AA. 

Variable  Resistance 

FIG.  i 

leads  are  to  be  reversed  at  the  wire.  Never  reverse  or  disconnect 
voltmeter  leads  at  the  meter,  while  the  other  ends  of  the  leads  are 
attached  to  live  terminals]  a  short  circuit  is  likely  to  result. 
Always  reverse  or  disconnect  the  leads  at  the  point  where  they  are 
attached  to  the  live  terminals.  Great  precaution  must-  also  be 
taken  with  low-reading  voltmeters,  as  they  are  easily  burnt  out. 
To  determine  an  unknown  low  voltage,  a  range  somewhat  above 
the  line  voltage  should  first  be  tried  and  a  reading  taken;  lower 
ranges  can  then  be  tried  until  the  proper  one  has  been  determined. 
Having  cetermined  the  proper  range  upon  the  voltmeter  and 
adjusted  the  variable  resistance  so  that  the  current  flowing  is 
exactly  the  desired  value,  read  the  voltmeter.  Move  the  volt- 
meter leads  to  include  12  inches  of  wire  and  repeat.  Continue  this 
operation  until  the  drop  over  the  entire  length  of  copper  wire 


TESTING  OF  ELECTRICAL    MACHINERY 


FIG.  2 


has  been  determined  and  then  do  the  same  with  the  remaining 
three  wires.  Raise  the  current  to  |  and  finally  to  the  full 
value  of  the  current-carrying  capacity  of  the  system  and  measure 
the  fall  in  potential  along  each  wire.  Record  readings  in  neatly 
tabulated  form  and  calculate  the  resistance  of  each  wire  for  each 
value  of  current.  Calculate  how  much  power  was  used  in  each 
wire  for  each  value  of  current.  Determine  the  average  value 
of  the  resistance  per  mil  foot  for  each  material  and  with  the  value 
obtained  calculate  the  resistance  of  coo  feet  of  No.  10  B.  &  S. 
wire,  obtaining  the  diameter  of  No.  10  wire  from  a  wire  table. 
(b)  Determine  the  hot  and  cold  resistance,  total  watts  and 
_Q— i  economy  for  a  50-  and  a  100- 

watt  Gem  or  metallized  fila- 
ment lamp,  and  for  a  25-watt 
Mazda  or  tungsten  lamp. 

The  filament  used  in  Gem 
lamps  is  of  carbon  which  has 
undergone  various  heat  treat- 
ments, causing  it  to  assume 
some  of  the  properties  of  metals,  the  most  important  being  a  posi- 
tive resistance  temperature  coefficient.  Carbon  has  a  negative 
coefficient,  or,  its  resistance  decreases  with  rise  in  temperature. 
In  metallized  filament  lamps  the  hot  resistance  will  be  found 
somewhat  greater  than  the  cold  resistance  and  the  same  applies, 
but  to  a  more  marked  degree,  in  the  case  of  tungsten. 

To  determine  the  hot  resistance  of  the  lamps,  connect 
them  as  in  Fig.  2,  to  a  source  of  E.M.F.  of  about  115  volts,  in 
series  with  an  ammeter  of  about  2  amperes  range.  No  series 
resistance  is  required,  as  the  lamps  themselves  are  of  high  enough 
resistance  to  withstand  the  full  line  voltage.  Using  a  voltmeter 
with  a  range  of  150  volts,  determine  the  voltage  across  the  lamp 
burning.  Remove  the  voltmeter  and  read  the  ammeter.  Do 
not  read  the  ammeter  with  the  voltmeter  connected  across  the 
lamp,  as  the  current  indicated  by  the  ammeter  will  be  the  sum 
of  the  lamp  and  voltmeter  currents  and  not  the  lamp  current 
alone.  Take  a  set  of  readings  for  each  lamp  and  record  them 
in  a  log  as  shown  in  Table  I. 


FALL   OF  POTENTIAL 
TABLE   I 


Type  of 
Lamp. 

Candle 
Power. 

Volts. 

Amperes. 

Watts. 

Economy 
or  Watts 
per 
Candle 
Power. 

Resistance. 

Hot  by 
Drop  in 
Potential 

Cold  by 
Wheatstone 
Bridge. 

» 

Determine  the  cold  resistance  of  the  lamps  by  means  of  a 
Wheatstone  bridge. 

In  calculating  economy  or  watts  per  candle  power,  assume  that 
the  lamp  gives  its  rated  candle  power  at  the  voltage  used.  Values 
of  candle  power  of  the  lamps  used  will  be  furnished  by  instructors. 
The  economy  serves  as  a  measure  of  the  efficiency  of  a  lamp,  in 
that  a  lamp  is  usually  rated  as  using  so  many  watts  per  candle 
power;  a  lamp  using  1.2  watts  per  candle  power  is  evidently 
more  efficient  as  a  light  producer  than  one  using  3  watts  per 
candle  power. 

Curves,  (a)  Plot  the  results  obtained  for  each  wire  on  one 
sheet  of  cross-section  paper,  plotting  voltmeter  readings  as  ordinates 
and  lengths  as  abscissae. 

Conclusions.  How  does  drop  in  potential  along  a  wire  vary  ? 
Why  are  the  curves  straight  lines?  What  might  cause  them 
to  be  slightly  concave  upward  ?  Which  metal  is  the  better 
conductor  and  which  best  adapted  for  use  as  resistance  wire 
in  rheostats  and  why  ?  Why  did  the  wires  expand  during  the 
experiment  ?  How  do  the  values  for  resistance  per  mil  foot  and 
for  the  resistance  for  1000  feet  of  No.  10  B.  &  S.  wire  compare 
with  values  obtained  from  wire  tables?  How  do  you  explain 
the  difference  in  the  hot  and  cold  resistances  of  the  lamps  used  ? 
How  do  the  efficiencies  of  the  different  types  of  lamps  compare? 
Explain  briefly  the  principle  and  operation  of  the  Wheatstone 
bridge. 


EXPERIMENT   II 

Measurement  of  Armature  Circuit  and  Shunt  Field  Resistances. 

(a)  The  resistance  of  an  armature  circuit  is  made '  up  of  the 
resistance  of  the  conductors  upon  the  armature,  the  brushes, 
the  brush  contacts,  and  the  cables  leading  from  the  brushes  to 
the  machine  terminals.  In  a  well-designed  motor  or  generator 
the  armature  circuit  resistance  is  made  as  low  as  is  consistent 
with  the  size  of  the  machine,  in  order  to  cut  down  the  amount 
of  energy  dissipated  as  heat.  The  rate  of  production  of  heat 
in  the  armature  is  given  by  the  formula,  watts  =  PR.  A  motor 
or  a  generator  is  designed  to  carry  a  certain  maximum  value  of 
armature  current  and  this  then  fixes  the  value  for  /,  so  that  to 
keep  the  amount  of  heat  generated  in  the  armature  low,  the 
resistance  must  be  made  as  small  as  is  commercially  practical. 

The  armature  conductors  being  always  of  copper,  their  resist- 
ance will  be  independent  of  the  current  except  for  heating.  The 
same  applies  to  the  machine  leads,  the  resistance  of  which 
is  very  small.  The  brush  contact  resistance  is  the  resistance  of 
the  surface  contact  between  the  carbon  brushes  and  the  com- 
mutator. This  resistance  is  quite  appreciable  and  decreases 
with  increase  of  current  strength;  it  decreases  as  the  mechanical 
pressure  between  the  brush  and  the  commutator  increases.  The 
resistance  of  the  brushes  themselves  is  insignificant. 

Determine  the  full  load  armature  current  of  the  machine,  whose 
armature  circuit  resistance  is  to  be  measured,  from  its  name- 
plate  data;  connect  the  armature  in  series  with  an  ammeter,  lamp 
board  and  a  variable  resistance,  all  of  suitable  current  carrying 
capacity,  as  shown  in  Fig.  3.  Close  the  line  switch  and  adjust 
the  current  to  £  of  the  full-load  value. 

Using  first  the  highest  range  of  the  voltmeter,  attach  the  leads 

6 


ARMATURE  CIRCUIT  AND  SHUNT  FIELD  RESISTANCES       7 

to  the  two  terminals  of  the  machine  (corresponding  to  position 
i  in  Fig.  3)  and  read  the  meter.  •  If  the  reading  is  within  the 
next  lower  range,  detach  the  leads  from  the  machine  terminals 
and  shift  to  the  lower  range.  When  a  suitable  range  has  been 
found  upon  the  voltmeter,  so  that  the  deflection  is  a  large  one, 
read  simultaneously  ammeter  and  voltmeter  and  record  the 


FIG.  3 


readings  in  a  log  as  in  Table  II.  From  the  readings  taken 
with  the  voltmeter  leads  attached  as  in  position  i,  the  resistance 
of  the  entire  armature  circuit  can  be  calculated.  Leave  one 
of  the  voltmeter  leads  upon  one  of  the  machine  terminals  and 
starting  from  the  one  employed,  trace  the  cable  which  leads  to 
the  machine.  Find  the  brush  to  which  this  cable  is  attached 
and  connect  the  other  voltmeter  lead  to  some  convenient  place 
upon  it  (position  2).  After  having  adjusted  the  current  to  its 


TESTING   OF   ELECTRICAL  MACHINERY 


proper  value  read  the  voltmeter.  From  this  reading  the  resistance 
of  one  of  the  machine  leads  can  be  determined.  Then  shift 
the  voltmeter  leads  successively  to  positions  3,  4,  5,  and  6,  reading 
the  meters  as  before.  These  readings  should  be  taken  as  rapidly 
as  possible,  so  that  the  armature  shall  not  heat  up  too  much. 
Repeat  this  operation,  using  2/3  and  full-load  armature  current, 
respectively.  The  sum  of  readings  2,  3,  4,  5,  and  6  entered  in 
column  7  of  the  log  should  equal  the  values  in  column  i,  the 
measured  drop  of  the  entire  circuit.  Calculate  the  values  of 
resistance  and  tabulate  in  a  form  similar  to  Table  II. 

TABLE   II 


Volts. 

I 

2 

3 

4 

5 

6 

7 

Difference 

Across 
entire 
Arma- 
ture. 
Circuit. 

Across 
ends  of 
one 
Machine 
Lead. 

Across 
one 
Brush 
Contact 

Across 
Arma- 
ture. 

Across 
second 
Brush 
Contact. 

Across 
ends  of 
second 
Machine 
Lead. 

Sum  of 
Drops  2, 
3.  4.  5 
and  6. 

i  and  7. 

Caution.  Great  caution  must  be  exercised  in  obtaining  the 
voltage  across  the  various  parts.  In  order  to  obtain  large  readings 
upon  the  voltmeter  it  will  be  necessary  to  shift  from  one  low 
reading  range  to  another,  so  that  it  must  constantly  be  borne  in 
mind  that  a  voltmeter  needle  is  easily  bent  or  the  instrument 
entirely  burnt  out,  if  a  high  potential  is  applied  to  a  low  range. 

To  determine  brush  drop,  connect  one  voltmeter  lead  to  the 
place  upon  the  brush  or  brush  stud  used  in  determining  the  drop 
across  the  machine  leads  and  hold  the  other  lead  on  the 
commutator  bar  directly  under  the  brush;  do  not  touch  the 
terminals  of  the  voltmeter  leads  against  the  brushes.  When 
determining  the  drop  across  the  armature  winding  hold  the 
voltmeter  leads  to  commutator  segments  which  are  under 
brushes  of  opposite  polarity,  again  being  careful  not  to  touch 


ARMATURE   CIRCUIT  AND   SHINT    FIELD   RESISTANCES        9 

the  terminal  on  the  commutator,  against  the  brush  while  reading 
the  voltmeter. 

Do  not  permit  the  armature  to  rotate,  as  a  counter  electro- 
motive force  is  generated  which  decreases  the  current  and  causes 
^n  apparent  increase  in  the  resistance  of  the  armature. 

Caution.  When  it  becomes  necessary  to  open  the  armature 
circuit  be  sure  that  the  voltmeter  leads  have  been  entirely  removed 
before  the  switch  is  opened.  A  large  amount  of  magnetic  energy 
is  stored  up  in  the  armature  while  current  is  flowing  and  when 
the  circuit  is  opened  this  energy  must  be  dissipated.  If  a  volt- 
meter is  connected  across  the  armature  terminals  a  high  protential 
will  be  applied  to  it,  due  to  the  self-induction  of  the  armature. 
This  is  quite  likely  to  bend  the  needle  of  the  voltmeter  and  may 
even  burn  it  out.  Therefore  never  suddenly  open  a  circuit  con- 
taining much  self-induction  while  a  voltmeter  is  connected  across 
any  part  of  it. 

(b)  The  function  of  a  shunt  field  is  to  provide  a  certain  number 
of  ampere-turns  for  the  magnetic  circuit  and  this  can  be 
accomplished  either  by  using  a  small  number  of  turns  carrying  a 
comparatively  large  current,  or  a  large  number  of  turns  with  a 
small  current  flowing  through  them.  Generally  a  large  number 
of  turns  is  preferred,  as  this  gives  a  high  field  resistance  and 
a  comparatively  small  consumption  of  energy  in  the  field  coils. 
A  shunt  field  is  always  so  built  that  it  may  be  safely  placed 
across  a  line  of  the  same  voltage  for  which  the  machine  was 
designed. 

It  must  be  remembered  that  a  shunt  field  possesses  consider- 
able self-induction,  so  that  when  a  difference  of  potential  is  applied 
to  it,  the  current  does  not  immediately  assume  its  final  value,  but 
builds  up  slowly.  This  is  due  to  the  counter-electromotive  force 
of  self-induction  which  is  set  up  while  the  current  is  increasing. 
This  "  building  up  "  of  the  current  can  be  seen  very  nicely  by 
watching  the  ammeter  as  the  line  switch  is  closed. 

Connect  the  shunt  field  whose  resistance  is  to  be  measured, 
in  series  with  an  ammeter  of  proper  capacity,  to  a  source  of  E.M.F. 
equal  to  that  for  which  it  was  designed;  the  connections  to  be  as 


10 


TESTING   OF  ELECTRICAL   MACHINERY 


shown  in  Fig.  4.  Close  the  line  switch  and  using  a  voltmeter 
of  the  proper  range,  determine  the  difference  of  potential  between 

the  ends  of  the  field.  Remove  the 
voltmeter  and  determine  the  current 
value.  Then  open  the  circuit  slowly, 
drawing  out  the  arc;  be  sure  the  volt- 
A/  meter  is  disconnected. 

Also  determine  the  resistance  of 
the  field,  using  a  Wheatstone  bridge. 
Conclusions.  Explain  why  arma- 
ture resistances  are  made  low  and 
shunt  field  resistances  high.  How 
do  the  resistances  of  the  various  parts 
of  the  armature  circuit  vary  with 
current?  Why  is  it  unsatisfactory  ;o 

determine  the  armature  circuit  resistance  with  the  ordinary 
Wheatstone  bridge.  Why  should  an  inductive  circuit  never  be 
opened  if  a  voltmeter  is  connected  across  any  portion  of  it  ? 


FIG.  4 


EXPERIMENT  III 

The  Shunt  Generator,  (a)  Preliminary  Work  with  a  Genera- 
tor; (b)  Magnetization  Curve  of  a  Shunt  Generator;  (c)  External 
Characteristic  of  the  Shunt  Generator. 

(a)  The  student  should  first  familiarize  himself  with  the  com- 
ponent parts  of  the  machine  assigned  and  look  over  their  construc- 
tion and  relation  to  one  another,  in  order  to  get  a  general  idea  of 
the  generator.  The  following  table  of  data  and  dimensions  is  then 
to  be  completely  filled  out. 

GENERAL 

Type  of  generator shunt    Current I . 

or  compound  K.  W.  Output 

Rated  voltage Speed   

ARMATURE 
Type ring  or  drum     Active  length 

Number  of  sections Circumference  in  feet 

Conductors  per  section Peripheral  speed  in  ft.  per  min. 

Total  number  of  conductors.  . . 

COMMUTATOR 

Active  length Average  voltage  between  bars  . 

Total  length Width  of  bars 

Circumference  in  feet Width  of  insulation 

Peripheral  speed  in  ft.  per  min. .      Kind  of  brushes 

Total  number  of  bars Brush  area  per  set 

Number  of  bars  between  brushes  Current  density  in  brushes,  in 

of  opposite  polarity amperes  per  sq.  in 

FIELDS 

Number  of  main  poles Pole  width 

Number  of  commutating  poles . .      Pole  length 

Pole  arc  in  degrees Width  of  pole  shoe 

n 


12  TESTING   OF   ELECTRICAL   MACHINERY 

A  shunt  generator  can  readily  be  distinguished  from  a  com- 
pound generator  by  examination  of  the  field  coils.  A  shunt  field 
is  wound  of  relatively  small  wire  and  there  will  be  only  two  ter- 
minals on  each  field  spool.  A  compound  generator  has  a  series 
winding  of  large  wire  or  copper  strip  in  addition  to  its  shunt  wind- 
ing, so  that  each  field  spool  will  have  two  large  additional  taps 
for  the  series  field  current.  The  remainder  of  the  general  data 
can  be  determined  from  the  name  plate. 

The  type  of  armature  can  generally  be  determined  by  exam- 
ining the  back  connections.  A  ring  winding  is  one  in  which  the 
wire  is  wound  around  an  annular  ring,  so  that  there  are  conduc- 
tors both  on  the  outside  and  the  inside  of  the  ring.  In  a  drum 
winding  all  of  the  conductors  are  upon  the  other  periphery.  An 
armature  section  can  be  defined  as  that  part  of  the  winding,  which 
starts  at  one  commutator  bar  and  ends  at  another.  The  number 
of  sections  upon  the  armature  can  sometimes  be  determined  from 
the  back  connections,  the  number  of  slots  and  the  connections 
to  the  commutator.  The  number  of  sections  will  never  be  less 
than  the  number  of  commutator  bars  and  will  generally  be  some 
multiple  or  sub-multiple  of  the  number  of  slots.  If  the  manu- 
facturer's data  is  at  hand  the  number  of  sections  can  be  taken 
therefrom  and  then  verified.  The  number  of  conductors  per 
section  will  probably  have  to  be  taken  from  the  manufacturer's 
winding  data.  The  active  length  of  the  armature  will  be  equal 
to  the  width  of  the  pole  shoe  (i.e.,  the  length  of  the  pole  shoe 
parallel  to  the  shaft)  plus  twice  the  length  of  the  air  gap. 

The  active  length  of  the  commutator  can  be  regarded  as  the 
distance  occupied  by  the  brushes  plus  the  spaces  between  them, 
measured  parallel  to  the  shaft.  A  commutator  is  always  made 
longer  than  this  to  permit  clearance  and  end  play.  In  deter- 
mining the  current  density  of  the  brushes,  remember  that  brush 
sets  always  operate  in  pairs.  In  a  bi-polar  generator  each  brush 
set  carries  the  total  current  of  the  machine.  The  pole  w'dth  is 
the  distance  parallel  to  the  shaft.  The  pole  length  is  measured 
out  radially. 

Before  a  generator  is  to  be  operated,  certain  conditions  should 


THE    SHUNT    GENERATOR  13 

be  noted,  particularly  in  a  new  machine  or  one  which  has  not  been 
run  for  some  time.     The  things  to  be  noted  are  as  follows: 

(1)  Amountof  oilinthebearingsorcups.  With  self -oiling  bearings 
it  is  advisible  to  note  whether  the  oil  rings  turn  easily  and  dip  into 
oil.     This  can  be  readily  seen  by  turning  the  rings  over  by  hand. 

(2)  Condition  of  the  brushes  and  commutator.     The  commu- 
tator should  be  clean  and  fairly  bright.     If  it  is  not,  it  can  be 
polished  while   revolving  with    fine  sandpaper  on  a  flat  wooden 
block  or  a  polishing  stone.     Do  not  use  emery.     The  brushes 
should  make  good  contact  over  their  whole  bearing  surface. 

(3)  Brush  Pressure.     This  should  be  from  one  and  a  half  to 
two  pounds  per  square  inch  of  contact  surface. 

(4)  The  armature  should  turn  easily,  either  by  hand  in  the 
case  of  a  small  machine  or  by  means  of  a  lever  in  a  large  one. 
If  the  armature    turns  hard,  either  the  bearings  are  in  bad  con- 
dition or  the  alignment  of  the  shaft  in  the  bearings  is  not  good. 

(b)  A  generator  is  a  device  which  transforms  mechanical 
energy  into  electrical  energy.  Its  operation  is  based  upon  the 
fact  that  when  a  conductor  is  moved  in  a  magnetic  field  so  as  to 
cut  lines  of  force,  an  E.M.F.  is  induced  in  it.  In  a  generator  a 
number  of  copper  wires  or  bars  are  mounted  upon  a  cylindrical 
iron  core,  and  this  armature,  when  rotated  in  a  magnetic  field, 
generates  a  voltage.  The  armature  inductors  are  properly 
interconnected,  so  as  to  add  their  individual  E.M.Fs.  and  are 
also  connected  to  the  commutator  which  rectifies  the  alternating 
voltages  induced  in  them. 

The  fundamental  equation  of  the  generator-  can  be  written  as 
follows : 

(/>XNXnX2p 

where  Eg  is  the  voltage  generated  by  the  armature; 
<£  =  the  flux  per  pole  in  lines  of  force; 

n  =  total  number  of  inductors  upon  the  armature; 

p  =  number  of  pairs  of  poles; 

c  =  number  of  circuits  in  parallel  upon  the  armature. 


14 


TESTING  OF   ELECTRICAL  MACHINERY 


The  derivation  follows  if  we  remember  that  when  an  inductor 
cuts  io8  lines  of  force  per  second,  an  E.M.F.  of  one  volt  is  induced 
between  its  ends. 

The  magnetic  field  of  a  generator  is  produced  by  the  ampere- 
turns  of  the  field  windings  according  to  the  formula, 

Magnetomotive  force  =.4^X  ampere-turns, 


Magneto- motive  Force 
FIG.   5 

and  this  magnetomotive  force  acting  over  the  reluctance  of  the 
magnetic  circuit  produces  a  flux  according  to  the  relation 

M.M.F. 

— r-         — . 

reluctance 

The  curve  showing  the  relation  between  flux  and  M.M.F.  is 
shown  in  Fig.  5;  it  is  seen  that  at  low  values  of  M.M.F.  a  small 
increase  in  M.M.F.  produces  quite  a  large  increase  in  flux.  As 
the  field  becomes  stronger  the  flux  curve  begins  to  bend  over 


THE   SHUNT   GENERATOR 


15 


more  and  more,  so  that  for  large  additions  of  M.M.F.  there  is 
only  a  small  increase  in  the  flux;  when  in  this  state  the  iron  is  said 
to  be  saturated. 

In  the  case  of  a  generator,  in  determining  the  magnetization 
or  saturation  curve,  we  make  use  of  the  fact  that  magnetomotive 
force  is  directly  proportional  to  the  field  amperes,  since  the  number 
of  field  turns  is  constant.  Furthermore,  in  Eq.  (6),  if  the  speed  is 
a  constant  quantity,  the  generated  voltage  will  be  a  measure  of 
the  flux,  all  the  other  quantities  being  constants.  The  magneti- 
zation curve  of  the  generator  may  then  be  plotted  between 
generated  volts  and  field  current. 

It  is  preferable  that  the  field  of 
the  generator  be  separately  excited, 
and  a  very  convenient  method  for 
doing  this  is  the  so-called  potentio- 
meter method.  This  consists  of  a 
rheostat,  with  a  sliding  contact,  placed 
across  the  D.C.  line  as  shown  in  Fig. 
6.  By  adjusting  the  slider  5,  any 
desired  voltage  can  be  had  across  the 
leads  PQ,  from  zero  up  to  the  value  of 
the  supply  voltage.  If  the  leads  PQ 
are  applied  to  a  field  it  is  possible  to  vary  the  field  current  gradu- 
ally from  zero  to  its  maximum  value. 

Rheostats  are  usually  made  of  tapering  sizes  of  wire  (shown 
diagrammatically  in  Fig.  6),  in  which  case  it  is  necessary  that  the 
field  be  put  in  parallel  with  the  fine  wire  end.  The  coarse  wire 
will  then  carry  the  field  current  plus  the  current  flowing  through 
that  portion  of  the  resistance  which  is  in  parallel  with  the  field. 

The  advantage  of  the  potentiometer  method  may  be  seen  from 
the  following  numerical  example.  The  shunt  field  of  a  generator 
is  to  be  separately  excited  from  a  2 50- volt  line.  The  resistance  of 
the  field  is  100  ohms  and  an  external  resistance  of  150  ohms  is 
available.  Using  the  resistance  in  series  with  the  field,  it  is  pos- 
sible to  vary  the  current  from  2.5  amperes  to  i  ampere  as  a  mini- 
mum; if  values  below  i  ampere  are  desired,  a  larger  external 


FIG.  6 


16 


TESTING  OF    ELECTRICAL    MACHINERY 


resistance  is  necessary.     Used,  however,  as  a  potentiometer,  the 

current  may  be  varied  from  2.5  to  zero  amperes. 

Excite  the  field  of  the  machine  to  be  used,  from  a  source  of 

E.M.F.  greater 
than  the  rated 
voltage  of  the 
machme> 


FIG.  7 


potentiometer 
|^  connection  as  is 
shown  in  Fig.  7. 
(Note  resistance 
of  rheostat  used 


on  log.)  For  a  ii5-volt  generator,  excite  the  field  from  a  23o-volt 
line,  etc.  With  the  line  switch  open,  rotate  the  machine  at  its 
rated  speed  bymeans  of  aprime 
mover  whose  speed  can  be  held 
constant;  place  a  voltmeter  of 
suitable  range  across  the 
brushes  of  the  machine.  With 
no  current  flowing  through  the 
field  of  the  machine,  it  will  be 
found  that  a  small  voltage  is 
generated.  This  is  due  to  a 
small  amount  of  residual  mag- 
netism retained  from  a  former 
excitation. 

Now  close  the  line  switch 
and  send  a  small  current 
through  the  field  of  the  gen- 
erator, making  sure  that  its 
direction  is  such  as  to  increase 
the  voltmeter  reading.  Then 
keeping  the  speed  of  the  ma- 
chine constant,  take  about  10 
readings,  raising  the  voltage 


Magnetization  Curve 


Field  Current 

FIG.  8 

50  per  cent  above  its  rated  value.     Then  decrease  the  value  of 
current  in  similar  steps  until  it  is  again  zero.     It  will  be  found 


THE  SHUNT    GENERATOR  17 

that  the  descending  curve  is  higher  than  the  ascending,  or  in  other 
words  that  the  same  field  current  gives  a  higher  generated  voltage 
descending  than  ascending.  This  is  due  to  hysteresis  or  the 
retentiveness  of  the  magnetic  path.  The  curve  obtained  is 
shown  in  Fig.  8. 

Caution.  Because  of  hysteresis,  great  care  must  be  taken 
when  on  the  ascending  curve,  to  always  bring  the  magnetizing 
current  up  to  the  value  at  which  a  reading  is  to  be  taken.  If  the 
desired  value  should  be  exceeded  and  the  current  subsequently 
reduced  to  it,  the  readings  obtained  would  lie  on  the  descending 
curve.  In  case  the  desired  value  of  field  current  is  exceeded, 
adjust  the  current  to  the  proper  value,  open  the  field  circuit 
slowly  for  an  instant  and  then  let  the  current  build  up  again. 
The  opposite  applies  to  the  descending  curve. 

After  obtaining  the  readings  for  the  magnetization  curve,  de- 
termine the  shunt  field  resistance,  using  the  same  connections  as 
before  and  measuring  the  voltage  across  the  field  at  the  machine. 

(c)  A  shunt  generator,  as  its  name  implies,  is  one  in  which  the 
field  is  shunted  or  connected  direc'r/  across  the  armature.  Thus 
a  part  of  the  armature  current  is  diverted  from  the  external  circuit 
and  sent  through  the  field.  As  has  been  seen  before,  the  resist- 
ance of  the  shunt  field  is  made  high  in  order  that  only  a  small 
percentage  of  the  current  output  of  the  generator  may  be  used 
in  this  part  of  the  machine:  the  power  expended  in  the  field 
circuit  is  lost  as  heat,  and  being  a  constant  loss,  every  effort  is  made 
to  reduce  it  to  as  low  a  value  as  is  practically  possible. 

It  was  mentioned  in  part  (a)  that  even  with  no  field  current 
a  small  flux  due  to  residual  magnetism  traverses  the  armature, 
which  the  conductors  cut  as  they  rotate.  If  the  shunt  field  is 
connected  across  the  brushes,  the  small  E.M.F.  generated  by  the 
armature,  causes  a  small  current  to  flow  through  the  field  winding, 
which  strengthens  the  residual  field.  This  increased  field  induces 
a  higher  E.M.F.  in  the  armature,  which  in  turn  causes  more 
field  current,  so  there  results  a  stronger  field,  and  so  on  until 
the  excitation  reaches  its  proper  value.  This  process  is  called 
"  building  up."  It  is  quite  evident  that  if  there  is  no  residual 
magnetism  present,  as  may  sometimes  occur,  that  the  machine 


18  TESTING   OF  ELECTRICAL  MACHINERY 

cannot  "  build  up  "  any  voltage.  In  this  case  the  field  must 
be  excited  by  some  exterior  source  of  power.  Another  possibility 
of  trouble  is  reversed  field  connections,  so  that  whatever  voltage 
is  generated  due  to  residual  magnetism,  causes  current  to  flow 
through  the  field  coils  in  such  a  direction  as  to  weaken  the  residual 
magnetism  instead  of  strengthening  it.  To  test  for  this,  open 
the  field  and  determine  the  voltage  due  to  residual  magnetism. 
Then  close  the  field  and  notice  if  the  voltmeter  drops  toward 
zero.  In  this  case  reverse  the  field  connections.  If  there  is  a 
short  circuit  in  the  external  circuit  the  machine  will  not  build 
up,  nor  can  it  if  the  field  connections  are  open.  If  the  residual 
magnetism  is  weak,  increasing  the  pressure  of  the  brushes  will 
often  start  a  machine  to  build  up  its  voltage. 

The  external  characteristic  of  a  shunt  generator  is  the  curve 
which  shows  the  relation  between  terminal  voltage  and  external 
current.  If  we  have  a  generator  rotating  at  rated  speed  and 
allow  it  a  definite  field  current,  it  will  generate  a  certain  E.M.F. 
If  the  external  circuit  is  closed  through  a  resistance,  a  current 
will  flow  through  the  armature  and  the  external  circuit,  depending 
upon  the  value  of  the  voltage  generated  and  the  resistance  of  the 
external  circuit.  It  has  been  seen  before  that  when  current 
flows  through  an  armature,  there  is  a  drop  of  potential,  so  that 
some  of  the  voltage  induced  by  the  generator  will  be  used  up  in 
the  armature  and  the  remainder  in  the  load  circuit.  We  may 
write  this  in  the  form  of  an  equation  as  follows: 

Eg=Et  +  IaRa, (7) 

where  Eg= voltage  generated  by  the  machine; 
Et  =  terminal  voltage; 
7a= current  flowing  through  the  armature; 
Ra= resistance  of  the  armature  circuit. 

As  more  and  more  load  is  placed  upon  the  machine  the  ter- 
minal voltage  is  decreased  due  to  the  increased  IR  drop  in  the 
armature.  In  addition  to  this  there  is  also  armature  reaction  to 
be  taken  into  account,  the  effect  of  this  being  generally  to  weaken 
the  field  and  thus  reduce  the  generated  voltage.  Furthermore, 


THE   SHUNT   GENERATOR 


19 


the  shunt  field,  being  also  connected  across  the  terminals  of  the 
machine,  receives  less  and  less  current  due  to  the  terminal  voltage 
decreasing  which  still  further  weakens  the  flux.  It  should  be  care- 
fully noted  that  the  decrease  in  shunt  field  current,  is  a  result  of 


/  Magnetization  Curve 

,  and 

I  External  Characteristic 


External  Current. 


Field  Current. 

FIG.  9 


increased  armature  IR  drop  and  armature  reaction,  for  if  the 
terminal  voltage  had  not  fallen  due  to  these  two  phenomena,  the 
shunt  field  current  would  have  remained  constant. 

It  is  evident  from  the  above  that  the  greater  the  armature 
resistance  the  larger  will  be  the  armature  JR  drop  and  the  more 
the  voltage  will  fall  off  with  the  load;  armature  reaction  should 
also  be  kept  as  small  as  possible. 

As  the  external  resistance  is  decreased  more  and  more,  a  point 
is  reached  where  the  external  current  no  longer  increases  but 
actually  decreases.  If  the  external  resistance  is  still  further 
decreased  the  current  'continues  to  decrease  until  when  dead 
short  circuit  is  reached  there  is  only  a  small  current  flowing. 
The  E.M.F.  generated  in  the  armature  under  such  short  circuit 
is  due  to. the  residual  flux  and  it  is  nearly  all  used  up  as  IR  drop 


20  TESTING   OF   ELECTRICAL   MACHINERY 

iii  the  armature,  the  terminal  voltage  being  practically  zero. 
The  reasons  for  the  curve  bending  back  on  itself  can  be  seen  by 
comparing  the  curve  with  the  magnetization  curve  as  in  Fig.  9. 
The  machine  with  no  external  current  flowing  is  operating  at 
a  point  upon  the  saturated  portion  of  the  curve.  When  the 
field  weakens  a  little  due  to  the  addition  of  load  as  stated  before, 
the  point  of  operation  is  then  lower  down,  but  as  the  field  is  still 
somewhat  saturated  the  change  in  flux  is  not  great.  However, 
when  the  machine  drops  down  to  the  straight  portion  of  the 
magnetization  curve,  where  a  small  change  in  field  current  causes 
a  large  decrease  in  flux,  then  the  generated  and  terminal  voltages 
fall  away  quite  rapidly  and  the  external  current  decreases. 

That  the  external  characteristic  bends  back  on  itself  may 
further  be  seen  if  we  consider  the  equation 


ft 

where  7e  =  cu  rent  flowing  through  external  circuit; 
It=  terminal  voltage  as  before; 
Re  =  resistance  of  the  external  circuit. 

Both  Re  and  Et  are  decreasing  quantities  throughout  the 
determination  of  the  external  characteristic  and  whether  Ie 
increases  or  decreases  depends  upon  their  relative  rates  of 
decrease.  The  operator  causes  Rc  to  decrease  as  he  chooses  by 
adding  load  to  the  machine,  but  the  rate  of  decrease  of  Et  is 
determined  from  Eq.  (7)  and  the  shape  of  the  magnetization 
curve.  At  first  as  Rc  is  decreased,  Et  does  not  decrease  relatively 
as  much  and  the  external  current  increases.  This  continues, 
Et  decreasing  at  a  relatively  increasing  rate,  until  finally,  the 
machine  having  reached  the  bend  in  the  magnetization  curve, 
Et  decreases  faster  than  Re  and  Ie  begins  to  decrease. 

To  determine  the  external  characteristic,  the  machine  should 
be  so  ad'usted  that  when  it  is  supplying  full  load  current  its 
terminal  voltage  is  at  its  rated  value.  However,  if  this  were 
done,  the  armature  current  would  reach  an  excessive  value  before 
the  curve  would  turn  back  on  itself,  and  it  is  therefore  advis- 
able to  obtain  a  curve  of  this  peculiar  form  by  starting  with  the 


THE   SHUNT   MOTOR  21 

machine  at  its  rated  voltage  at  no  load.  Such  external 
characteristic  is  illustrated  by  the  broken  curve  in  Fig.  9. 

According  to  modern  practice  nearly  all  large  generators  and 
most  small  ones  are  equipped  with  commutating  poles,  by  whose 
aid,  sparking  at  the  commutator  and  the  necessity  for  brush 
shifting  are  done  away  with.  The  presence  of  commutating 
poles  may  to  some  extent  alter  the  shape  of  the  external  char- 
acteristic so  that  it  is  preferable  to  use  a  generator  not  so  equipped 
to  determine  this  curve. 

Connect  the  machine  as  indicated  in  Fig.  10,  using  an  ammeter 
of  the  proper  range  in  the  field  and  one  in  the  external  circuit  whose 
range  will  equal  twice  the  rated  full  load  current  of  the  machine* 

Adjust  the  machine  to  normal  voltage  at  full  load  and  rated 
speed  and  shift  the  brushes  to  the  position  giving  best  com- 
mutation. Without  altering  the  field  rheostat  remove  the  load, 
and  after  making  certain  that  the  speed  is  still  at  its  rated  value 
read  the  voltmeter.  The  per  cent  rise  in  voltage  determines  the 
regulation  of  the  machine,  as  the  regulation  of  a  generator  is 
given  by  the  equation. 

,   ..         no  load  voltage  —  full  load  voltage 
Regulation  =  -  f  °- . 

full  load  voltage 

Shift  the  brushes  to  give  no  sparking  and  then  lower  the 
voltage  of  the  generator  at  no  load  to  its  rated  value,  and  after 
noting  that  the  speed  is  at  its 
proper  value    read  the  meters. 
(If  the  generator  used  is  equipped 
with  commutating  poles,  it  will 
not  be   necessary  to   shift   the 
brushes  if  they  were  properly  set 
to   start.)      Then   keeping   the  FIG 

speed    constant     and    without 

altering  the  field  resistance  or  shifting  brushes,  continue  adding 
load  in  the  form  of  lamps  or  water  rheostats  in  equal  incre- 
ments, each  time  taking  a  reading  upon  all  the  meters.  Continue 
this  until  the  voltage  is  low  enough  for  the  machine  to  be  com- 
pletely short  circuited.  Record  readings  and  calculate  the 
armature  IR  drop  as  indicated  in  Table  III. 


22  TESTING   OF   ELECTRICAL   MACHINERY 

TABLE  III 


After  this  test  (using  a  generator  not  provided  with  corn- 
mutating  poles),  bring  the  machine  to  rated  voltage  at  rated 
speed  with  no  load  upon  it,  adjusting  the  brushes  to  give  best  com- 
mutation. Then  add  full  load  and  shift  the  brushes  forward 
until  the  point  of  best  commutation  is  again  reached :  watch  the 
voltmeter  very  carefully  while  this  is  being  done.  Then  shift  the 
brushes  further  forward  and  then  backward  beyond  the  no-load 
neutral  point,  again  noting  the  voltmeter  and  also  sparking. 

Finally  measure  the  armature  resistance  for  several  values 
of  current  from  zero  to  the  maximum  value  used.  Plot  a  curve 
between  armature  current  and  armature  resistance  and  use  this 
curve  in  calculating  IR  drop  in  table. 

Curves.  Plot  the  magnetization  curve  of  the  machine  from 
the  results  obtained;  plot  the  external  characteristic  on  the  same 
sheet,  n^ing  same  scale  for  E.M.F. 

Conclusions.  What  are  the  advantages  of  the  potentiometer 
method  for  separately  exciting  a  field  ?  In  determining  the  mag- 
netization curve,  if  the  rheostat  used  had  been  connected  ; 
with  the  field,  what  would  the  lowest  obtainable  value  of  current 
have  been?  How  much  resistance  in  series  with  the  shunt  field 
would  have  been  necessary  to  obtain  the  lowest  value  of  field  cur- 
rent you  recorded?  What  does  the  magnetization  curve  show? 
Why  are  the  ascending  and  descending  curves  not  coincident? 
What  purpose  does  residual  magnetism  serve?  For  what 
reasons  might  a  generator  refuse  to  build  up  and  what  actic: 
necessary  to  remedy  the  trouble?  What  is  the  regulation  in 
per  cent  of  the  machine  tested?.  Explain  why  the  terminal 
voltage  of  a  generator  tends  to  fall  with  increase  of  load.  Why 
should  the  armature  resistance  of  a  generator  be  made  low? 
Explain  briefly  the  principles  upon  which  armature  reaction  and 
sparking  depend.  Why  does  the  presence  of  commutating  poles 
eliminate  sparking  end  the  necessity  of  shifting  brushes. 


EXPERIMENT  IV 

The  Compound  Generator,  (a)  Armature  Characteristic  of 
a  Shunt  Generator,  (b)  External  Characteristic  of  a  Compound 
Generator,  (c)  Effect  of  Operating  a  Compound  Generator  at 
Speeds  Higher  or  Lower  than  Rated  Value. 

(a)  We  have  seen  that  when  a  shunt  generator  is  loaded,  the 
terminal  voltage  falls  if  no  attempt  is  made  to  regulate  it,  the, 
decrease  being  due  to  IR  drop  in  the  armature  and  armature 
reaction.  As  a  result  of  these,  the  shunt  field  current  is  reduced, 
causing  a  still  further  decrease  in  voltage.  Referring  to  the 
equation  Ey=Et+IR+\t  is  evident  that  if  the  terminal  voltage 
£:  is  to  be  maintained  constant  while  IR^  is  increasing,  the  value 
of  Eg  must  be  increased.  Referring  to  Eq.  (6)  it  will  be  seen 
that  either  the  speed  or  the  field  flux  might  be  increased  and 
the  generated  voltage  thereby  raised.  Increasing  the  field  flux 
is  the  more  feasible,  as  the  prime  movers  used  for  driving  gene- 
rators are  built  to  maintain  a  constant  speed.  Then  as  load  is 
added  to  the  generator,  sufficient  additional  flux  must  be  provided 
in  order  to  raise  the  generated  voltage  enough  to  compensate  for 
armature  IR  drop  and  armature  reaction.  In  order  to  provide 
more  flux,  more  shunt  field  current  is  necessary,  so  that  the  no- 
load  shunt  field  current  will  be  smaller  than  the  full-load  field 
current.  There  must  then  be  some  external  resistance  in  series 
with  the  shunt  field  at  no  load,  which  can  be  cut  out  and  the 
shunt  field  current  thereby  increased  as  load  is  increased. 

The  armature  characteristic  is  the  curve"  showing  the  relation 
between  the  external  and  die  shunt  field  currents,  when  the  ter- 
minal voltage  is  maintained  constant  by  shunt  field  regulation. 
The  curve  is  usually  concave  upward  (as  is  shown  in  Fig.ii) 
due  to  the  fact  that  the  increase  in  flux  per  unit  increase  of  the 
shunt  field  current,  as  indicated  by  the  magnetization  curve, 


24 


TESTING  OF   ELECTRICAL   MACHINERY 


decreases  as  the  iron  approaches  saturation.  Accordingly  there 
is  needed  a  greater  increase  in  shunt  field  current  to  compensate 
for  a  given  value  of  armature/^  drop  and  armature  reaction 
at  high  loads  than  at  light  loads. 

Since  the  number  of  turns  upon  the  shunt  field  is  constant, 


No  Load  Shunt  FteU  Current 


Armature  Characteristic 

of  a 
Shunt  Generator 


I 

I 


FIG.  ii 


and  the  number  of  shunt  ampere  turns  therefore  always  pro- 
portional ta  the  shunt  field  current,  we  can  then  determine  the 
per  cent  increase  in  the  number  of  ampere  turns  from  no  load 
to  full  load. 

Connect  the  generator  to  be  tested  as  in  Fig.  10  and  start  at 
rated  voltage  and  speed  with  no  load  upon  the  machine.  Then  add 
load,  maintaining  rated  speed  and  adjust  the  shunt  field  resistance 
so  that  for  each  addition  of  load  the  voltage  is  brought  back  to 


THE   COMPOUND  GENERATOR  25 

its  rated  value.     Take  eight  readings  up  to  25  per  cent  over- 
load.    Use  log  as  in  Table  III. 

(b)  It  was  pointed  out  in  Exp.  Ill  that  a  shunt  generator,  when 
no  attempt  at  regulation  was  made,  decreased  its  terminal  voltage 
with  increase  of  load.  However,  if  sufficient  flux  was  added 
to  compensate  for  the  factors  causing  the  decrease,  then  the 
terminal  voltage  remained  constant.  In  part  (a)  it  was  seen 
that  the  necessary  increase  in  field  ampere-turns  could  be  obtained 
by  decreasing  the  resistance  of  the  shunt  field  circuit.  A  very 
simple  way  of  causing  an  increase  in  the  field  ampere-turns  is 
to  employ  the  external  current,  wThich  causes  the  terminal  voltage 
to  fall,  by  simply  passing  it  through  the  series  field  winding 
which  consists  of  several  turns  of  large  wire.  Such  a  machine 
is  called  a  compound  generator.  The  series  field  carries  all  or 
a  fixed  per  cent  of  the  current  output  of  the  machine  and  is  then 
in  series  with  the  external  circuit.  The  shunt  field  provides  the 
correct  no-load  flux  of  the  machine,  while  the  series  field  com- 
pensates for  the  loss  of  voltage  due  to  armature  reaction,  IR 
drop  in  the  armature,  and  IR  drop  in  the  series  field  itself.  If 
the  number  of  series  turns  is  more  than  sufficient  to  compensate 
for  all  the  losses  of  E.M.F.  then  the  terminal  E.M.F.  will  rise 
with  increase  of  load  current.  In  lighting  systems  and  isolated 
plants  the  overcompounding,  as  it  is  termed,  is  about  2  to  3  per 
cent,  while  in  railway  work  the  generators  are  usually  10  per 
cent  overcompounded,  the  E.M.F.  rising  from  perhaps  500  volts 
at  no  load  to  550  volts  at  full  load.  This  regulation  is  entirely 
automatic  and  almost  instantaneous.  Generally  machines  are 
built  with  more  than  enough  series  turns,  and  then  a  portion  of 
the  external  current  is  shunted  off  by  means  of  a  German  silver 
shunt,  so  that  by  varying  the  resistance  of  the  shunt  a  wide 
degree  of  compounding  can  be  obtained. 

From  the  data  obtained  in  part  (a)  the  number  of  series  turns 
necessary  to  convert  the  machine  there  used  to  a  compound 
generator,  can  be  calculated  if  the  number  of  turns  upon  the  shunt 
field  are  given,  as  we  then  know  the  actual  increase  in  the  ampere- 
turns  from  no  load  to  full  load.  By  dividing  the  increase  in  ampere- 


23  TESTING  OF  ELECTRICAL   MACHINERY 

turns  by  the  value  of  the  full-load  external  current  we  obtain 
the  minimum  number  of  series  turns  that  the  machine  requires. 

It  will  be  found  that  the  external  characteristic  of  a  compound 
generator  is  somewhat  convex  upward,  this  being  due  to  the 
shape  of  that  portion  of  the  magnetization  curve  upon  which 
the  machine  is  operated. 

In  studying  the  compound  generator  it  is  seen  that  there  are 
two  different  field  currents,  one  circulating  through  a  winding  of 
a  large  number  of  turns  of  fine  wire,  the  other  through  a  winding 
of  a  few  turns  of  large,  wire  or  strip,  so  that  there  must  then  be 
devised  some  way  of  expressing  one  in  terms  of  the  other.  This 
is  possible  if  we  know  the  ratio  of  series  to  shunt  turns,  and  this 
ratio  can  be  experimentally  determined  as  follows:  The  series 
field  of  the  machine  is  first  separately  excited  from  some  outside 
source  of  power  and  a  current  sent  through  it,  equal  to  the  full 
load  external  current  of  the  machine.  Then  with  the  machine 
rotating  at  rated  speed  the  voltage  across  the  armature  is  deter- 
mined. There  being  no  current  through  the  armature,  generated 
and  terminal  voltages  are  the  same.  Then  in  turn,  the  shunt  field 
is  separately  excited  and  a  current  is  sent  through  it  of  such  a 
value  that,  with  the  machine  again  rotating  at  rated  speed,  the 
terminal  voltage  is  the  same  as  before. 

In  both  cases  the  armature  cut  the  same  number  of  lines  of 
force,  since  it  was  rotating  at  the  same  speed  and  generating  the 
same  voltage.  The  number  of  ampere-turns  upon  the  field  must 
therefore  have  been  identical  in  both  cases,  so  that  we  may  write 


reluctance     reluctance' 

where  n\  =  number  of  shunt  turns; 
n2  =  number  of  series  turns; 
1  1  =  shunt  field  current  ; 
J2  =  series  field  current. 

Eliminating  constants  we  have 
h     n2 

—  =  —  =  A. 
/2       ^i 


THE   COMPOUND   GENERATOR 


27 


which  states  that 

series  turns, 
Equivalent  shunt  field  current  =  series  field  current X~r~ 

shunt  turns 


or 


Equivalent  shunt  field  current  =  series  field  current  XK. 


In  case  the  series  field  has  a  German  silver  shunt  and  the 
actual  ratio  of  turns  is  desired,  it  will  be  necessary  to  determine 
the  actual  current  flowing  through  the  series  field.  Where  the 
value  of  K  is  to  be  used  in  getting  the  total  magnetizing  current, 
it  is  better  in  determining  K,  to  assume  that  all  of  the  external 


FIG.  12 

current  flows  through  the  series  field.  The  value  thus  obtained 
will  be  less  than  the  actual  value,  but  it  makes  calculations  easier, 
since  to  obtain  total  magnetizing  current  we  need  only  multiply 
the  external  current  by  K  to  obtain  the  shunt  current  equivalent 
to  the  series  field  current. 

Thus  for  any  load  condition  we  may  add  to  the  shunt  field 
current,  the  corresponding  series  field  current  multiplied  by  K 
and  obtain  the  total  magnetizing  current  expressed  in  terms  of 
the  shunt  field  current.  If  the  same  generator  was  used  for  both 
parts  (a)  and  (b)  then  a  curve  plotted  between  external  current 
and  total  magnetizing  current  from  the  results  of  part  (b)  should 
be  very  nearly  the  same  as  the  armature  characteristic  of  part  (a). 

Connect  up  the  machine  as  in  Fig.  12  and  operating  it  at  rated 


28 


TESTING   OF  ELECTRICAL  MACHINERY 


speed ,  adjust  the  shunt  field  resistance  to  give  rated  no  load  volt- 
age. Then  with  no  further  adjustment  than  to  keep  the  speed 
constant,  add  load  to  the  machine.  Read  external  and  shunt 
field  currents  and  terminal  E.M.F.  taking  ten  readings  from  no 
load  to  25  per  cent  overload.  Use  a  log  as  shown  in  table  IV. 

TABLE    IV 


Shunt  Field 

Terminal 
Volts. 

External 
Current. 

Shunt  Field 
Current. 

Current 
Equivalent  to 
Series  Field 

Total 

Magnetizing 
Current. 

Speed. 

Current. 

i 

2 

3 

4 

5 

6 

(2)X"K" 

3+4 

(c)  It  is  of  considerable  importance  that  a  compound  generator 
be  operated  at  rated  speed  if  it  is  to  compound  as  intended.  If  a 
flat  compounded  generator  is  operated  at  a  speed  higher  than  rated, 
but  with  rated  voltage  at  no  load,  it  will  be  found  that  it  will 
overcompound  as  load  is  added.  If  it  had  been  operated  at 
lower  than  rated  speed,  it  would  have  undercompounded. 

We  have  seen  from  Eq.  (6)  that  Eg=k<if)N)  so  that  at  rated 
speed  there  is  necessary  at  no  load  a  certain  value  of  flux.  This 
is  shown  in  Fig.  13,  oc  being  the  value  of  the  shunt  field  current 
and  oi  the  value  of  the  flux  produced.  As  load  is  added  to  the 
machine  the  series  field  provides  a  certain  additional  number 
of  ampere-turns  shown  by  the  distance  cd.  The  machine  thus 
operates  between  the  points  p  and  q  on  the  magnetization  curve, 
the  flux  added  being  represented  by  the  dstance  ij.  This  amount 
of  flux  is  sufficient  to  compensate  for  the  loss  of  E.M.F.  due  to 
the  addition  of  load,  so  that  the  machine  is  flat  compounded,  as 
shown  in  the  full  line  curve  in  Fig.  14.  If  the  machine  is  to  be 
operated  at  a  speed  higher  than  rated,  it  follows  that  if  the  arma- 
ture is  to  generate  the  same  no-load  voltage,  the  same  amount 


THE  COMPOUND   GENERATOR 


29 


of  flux  must  be  cut  per  second,  so  that  if  the  speed  is  raised,  less 
flux  is  necessary.  Under  these  conditions  a  smaller  shunt  field 
current,  shown  as  oa,  is  sufficient,  so  that  the  machine  begins 
operating  at  the  point  m  on  the  curve,  the  value  of  the  flux  being 


Total  Magnetizing  Current 
Expressed  in  Terms  of 
Shunt  Field  Current 

FIG.  13 


When  full  load  is  put  upon  the  machine,  the  series  field  pro- 
vides the  same  addition  of  ampere-turns  as  before,  so  that  the 
distance  ab  is  equal  to  cd.  The  flux  thereby  added  is  shown  as 
gh,  which  is  more  than  ij  (the  amount  just  necessary  to  compensate 
for  loss  of  voltage)  which  causes  the  machine  to  overcompound 
as  shown  in  Fig.  14.  By  similar  reasoning  it  will  be  seen  that 
operating  below  rated  speed  requires  more  no-load  shunt  field 
current,  but  the  magnetic  circuit  is  then  more  highly  saturated 
so  that  the  series  field  is  unable  to  provide  sufficient  flux  to  com- 
pensate for  the  losses  in  potential  and  the  machine  undercom- 
pounds. 


30 


TESTING  OF  ELECTRICAL  MACHINERY 


Operate  the  machine  with  the  same  connections  as  in  part 
(b)  Sit  speeds  15  or  20  per  cent  above  and  below  rated  value,  begin- 
ning at  the  rated  value  of  terminal  voltage.  Take  about  eight 
readings  up  to  full  load. 


Rated  Value 
ofE.M.F^ 


ted\Sf)eed_ 


Rated 


External  Current 
FIG.  14 

Determine  the  value  of  K  for  the  machine  as  indicated  in 
part  (b).    The  connections  are  shown  in  Fig.  15. 


FIG.  15 


THE  COMPOUND  GENERATOR  31 

Curves.  Plot  the  armature  characteristic  and  to  the  same  set 
of  co-ordinates,  plot  a  curve  between  external  current  and  total 
magnetizing  current  as  obtained  in  part  (b).  To  another  set  of 
co-ordinates  plot  the  three  compound  characteristics  as  obtained 
in  parts  (b)  and  (c). 

Conclusions.  What  is  the  increase  in  ampere-turns  from  no 
load  to  full  load  in  per  cent  of  the  full  load  value,  in  the  shunt 
generator  operating  at  rated  speed?  If  the  number  of  shunt 
field  turns  can  be  obtained,  calculate  the  number  of  series  field 
turns  necessary  to  make  the  machine  flat  compounded  with  all 
of  the  external  current  passing  through  the  series  field.  Why 
should  the  resistance  of  a  series  field  be  made  low?  Explain 
why  a  flat  compounded  generator  overcompounds  or  under- 
compounds,  when  operated  at  higher  or  lower  than  rated  speeds 
respectively,  if  started  at  rated  no-load  voltage.  Would  you 
consider  the  operation  of  a  shunt  generator  satisfactory  for 
commercial  work  if  the  load  is  widely  varying  and  constant 
terminal  voltage  is  desired?  Does  the  compound  generator 
meet  the  difficulties?  Explain  why. 


EXPERIMENT   V 

The  Shunt  Motor,  (a)  Speed  Characteristics,  (b)  Commer- 
cial Efficiency  by  Brake  Test. 

From  a  structural  standpoint  the  shunt  motor  and  the  shunt 
generator  are  identical  and  it  will  also  be  seen  that  both  depend 
upon  the  same  phenomena  for  their  operation.  In  fact  any 
direct-current  dynamo-electric  machine  that  can  be  used  as  a 
generator  can  also  be  used  as  a  motor,  the  only  point  of  difference 
being  in  their  function  and  application.  A  generator  has  mechan- 
ical energy  supplied  to  its  shaft  and  this  energy  is  converted  into 
electrical  energy  by  the  rotation  of  the  armature  in  the  magnetic 
field.  In  the  case  of  the  motor,  electrical  energy  is  supplied 
to  its  field  and  armature  and  the  machine  converts  it  into 
mechanical  form  at  the  shaft. 

Motors  are  usually  classed  according  to  their  field  windings^ 
these  being  known  as  shunt,  series,  and  compound ;  these  types 
correspond  exactly  with  those  in  the  generator. 

The  operation  of  a  motor  involves  two  fundamental  phenom- 
ena. The  first  is,  that  when  a  conductor  carrying  current  is 
placed  in  a  magnetic  field,  a  force  is  developed  which  tends  to 
move  the  conductor  in  a  direction  at  right  angles  to  itself  and  to 
the  magnetic  flux.  This  force  is  directly  proportional  to  both 
the  strength  of  the  field  and  the  current  flowing  in  the  conductor. 
The  direction  in  which  the  conductor  moves  (for  obviously  it 
can  sweep  across  the  field  in  either  of  two  ways)  depends  upon 
the  direction  of  the  current  in  the  conductor  and  the  direction 
of  the  magnetic  flux. 

In  a  motor  we  have  a  number  of  conductors  distributed  around 
the  periphery  of  the  armature,  and  when  the  latter  is  placed  in 
a  magnetic  field,  some  of  the  conductors  develop  a  torque  when 
current  flows  through  them  and  move  at  right  angles  to  the  field, 

32 


THE    SHUNT  MOTOR  33 

thereby  rotating  the  armature.  As  they  move  out  of  the  field, 
others  take  their  places  and  in  turn  also  move  out,  so  that  the 
torque  is  continuous  and  the  armature  keeps  on  rotating. 

The  second  phenomenon  is  the  same  as  that  upon  which 
the  operation  of  a  generator  depends,  namely,  that  when  a  con- 
ductor, whether  carrying  current  or  not,  is  moved  in  a  magnetic 
field  so  as  to  cut  lines  of  force,  an  electromotive  force  is  generated 
in  it. 

In  the  case  of  the  generator,  as  soon  as  current  flows  in  the 
windings  of  the  armature,  a  torque  is  produced  which  tends  to 
move  the  conductors  in  a  direction  opposite  to  that  in  which  the 
arnature  rotates,  due  to  the  fact  that  the  conductors  are  carrying 
current  and  are  placed  in  a  magnetic  field.  That  is,  the  current, 
which  flows  through  the  conductors  as  a  result  of  their  cutting 
the  magnetic  field,  causes  a  torque  in  the  direction  opposite 
to  that  in  which  they  are  moving.  It  is  this  counter  torque 
which  the  prime  mover  must  overcome  in  order  to  continue  the 
rotation  of  the  armature. 

In  the  motor,  when  current  is  sent  through  the  armature 
winding,  a  reaction  takes  place  between  the  armature  conductors 
and  the  magnetic  field  and  the  armature  rotates.  As  soon  as 
it  begins  to  rotate,  due  to  the  fact  that  the  conductors  are  cutting 
the  lines  of  force  of  the  field,  an  E.M.F.  is  generated,  which  is 
in  the  opposite  direction  to  that  impressed  upon  the  armature. 
This  is  called  the  " counter"  E.M.F.  of  the  motor  and  evidently 
it  cannot  become  equal  to,  or  greater  than  the  impressed  E.M.F., 
so  long  as  the  machine  is  operating  as  a  motor,  for  then  no  current 
would  flow  and  the  armature  would  cease  rotating. 

It  follows  then,  that  the  generated  E.M.F.  of  the  generator 
and  the  counter  E.M.F.  of  the  motor  are  the  same  and  that  the 
terminal  E.M.F.  of  the  generator  becomes  the  impressed  E.M.F. 
of  the  motor.  In  Eq.  (7)  (Eg=Et+IRa),  we  saw  that  while 
the  machine  was  delivering  current  and  IR  therefore  positive, 
that  Eg  was  greater  than  Et,  while  if  no  current  flowed,  Eg  and 
Et  would  be  equal.  If  the  machine  operates  as  a  motor  and 
takes  current  (which  for  the  same  direction  of  rotation  could  flow 


34  TESTING  OF   ELECTRICAL    MACHINERY 

in  the  reverse  direction)  IR  would  be  negative.     Under  these  con- 
ditions Eg  is  called  the  counter  E.M.F.  and  is  evidently  less  than 
Et.     We  may  then  write 

e  =  Et-IR,    .     .     .....     (8) 

where  e  =  C.  E.M.F. 

By  transposition  vrc  c-lso  l.£,vc 

£-«+•**>     .......    (9) 

Pt-e 
and  ^£T>     ....... 

The  latter  form  is  really  Ohm's  law  for  a  motor  and  it  indi- 
cates that  the  armature  current  is  caused  to  flow  by  an  effective 
E.M.F.,  which  is  equal  to  the  difference  between  the  impressed 
and  the  counter  E.M.F.'s. 

Since  the  C.  E.M.F.  of  a  motor  is  the  same  as  the  generated 
E.M.F.  of  a  generator,  then  Eq.  (6)  is  also  that  for  the  C.E.M.F. 
of  a  motor,  so  that  we  have 

e=!HV_^  ..... 

8 


which  indicates  that  the  C.E.M.F.  depends  upon  the  flux  and  the 
speed. 

The  shunt  motor,  as  stated  before,  has  its  field  and  armature 
connected  in  parallel  across  the  supply  circuit.  Since  the  resist- 
ance of  the  shunt  field  is  constant  except  for  temperature  changes, 
the  field  current  and  therefore  the  flux  will  depend  upon  the 
line  difference  of  potential.  This  usually  has  a  constant  value 
and  as  a  result,  the  flux,  so  far  as  it  depends  upon  the  field  current, 
will  remain  constant,  i.e.,  independent  of  load. 

With  this  fact  in  mind,  let  us  now  consider  what  happens 
when  load  is  placed  upon  the  armature.  This  means  that  the 
armature  must  increase  its  torque  or  turning  effort  and  to  effect 
this,  there  must  be  a  greater  force  exerted  between  the  armature 
inductors  and  the  field  flux.  This  force  is  proportional  to  the 
product  of  field  flux  and  armature  current,  so  that  if  it  is  to  be 
increased  there  must  be  an  increase  in  the  armature  current, 


THE   SHUNT   MOTOR  35 

the  field  flux  being  constant.  Now  consider  Eq.  (10),  in  which 
Et  and  Ra  are  constant.  If  /a  is  to  increase,  e  must  decrease, 
and  in  order  that  this  may  be,  since  <£  is  constant,  the  speed  must 
fall.  This  brings  us  to  the  first  operating  characteristic  of  the 
shunt  motor,  namely,  that  as  the  load  is  increased,  the  speed  falls. 
Thus,  when  additional  load  is  put  upon  the  motor,  the  machine 
at  that  instant  not  developing  the  required  torque,  begins  to 
slow  up  a  trifle.  This  causes  the  C.E.M.F.  to  decrease,  more 
current  passes  through  the  armature  and  a  greater  torque  is 
developed.  This  continues  until  the  motor  exerts  the  required 
torque,  the  speed  becoming  constant  at  a  value  a  little  below 
what  it  was  before.  In  a  well-designed  shunt  motor  the  decrease 
in  speed  from  no  load  to  full  load  is,  however,  very  small,  so 
that  the  shunt  motor  is  often  called  a  constant-speed  machine. 
From  Eq.  (8)  it  is  evident  that  if  Ra  is  high,  the  drop  in  speed 
will  be  large,  so  that  to  obtain  good  speed  regulation  with  load, 
the  armature  resistance  must  be  low.  Substitution  of  some 
actual  values  in  Eq.  (9)  will  give  an  idea  of  the  relative  changes 
of  C.E.M.F.  and  armature  current  from  no  load  to  full  load. 
A  5  h.p.,  1 10- volt  shunt  motor  would  require  an  armature  current 
of  about  40  amperes  at  full  load  and  about  4  amperes  at  no  load. 
The  armature  circuit  resistance  of  such  a  motor  would  be  about 
0.25  ohm.  Using  no  load  values  in  Eq.  (9)  would  give 

110  =  109+4X0.25 

and  at  full  load  110  =  100+40X0.25. 

With  a  small  change  of  9  volts  in  the  C.E.M.F.  the  armature  cur- 
rent rose  from  4  to  40  amperes,  which  is  a  considerable  change. 
Let  us  now  consider  what  happens  when  resistance  is  inserted 
into  the  armature  circuit.  When  current  flows  through  the  resist- 
ance there  will  be  an  IR  drop  across  it,  so  that  the  E.M.F. 
applied  to  the  armature  terminals  will  be  the  difference  between 
the  line  E.M.F.  and  the  IR  drop  across  the  variable  resistance. 

Then  Et-IRVT=e+IRa (12) 

For  this  equation  to  be  satisfied,  e  must  decrease,  but  since 
e  =  K((>N,  in  which  </>  is  constant,  the  speed  must  fall.  This 


36  TESTING   OF   ELECTRICAL   MACHINERY 

in  one  sense,  is  equivalent  to  changing  the  armature  resistance, 
as  will  be  seen  if  we  rewrite  Eq.  (12)  in  the  form 


The  effect  of  adding  resistance  to  the  shunt  field  circuit  is 
just  the  reverse.  The  first  effect  produced  is  a  decrease  in  the 
shunt  field  current,  which  in  turn  decreases  the  flux  and  the 
C.E.M.F.  This  permits  more  armature  current  to  flow  and 
the  extra  torque  developed  accelerates  the  motor  until  the 
C.E.M.F.  has  built  up  to  such  a  value  that  normal  armature 
current  is  again  flowing.  If  the  armature  current  has  nearly 
the  same  value  in  both  cases  (i.e.,  before  and  after  resistance 
was  added  to  the  field)  then  the  value  of  the  C.E.M.F.  must  be 
nearly  the  same  for  both  cases  and  in  order  to  satisfy  this  condi- 
tion for  a  decrease  in  flux,  there  must  be  a  corresponding  increase 
in  the  speed.  To  operate  the  motor  at  a  higher  speed  requires  a 
slight  increase  in  torque,  so  that  with  the  weaker  field  the 
armature  current  must  increase  enough  to  cause  the  motor  to 
exert  a  little  greater  torque  than  before.  This  will,  however, 
require  only  a  slight  change  in  the  C.E.M.F.  as  was  noted  before. 
The  practical  limit  to  weakening  the  field  is  imposed  by  sparking 
at  the  brushes,  due  to  the  fact  that  with  the  weakened  field, 
armature  reaction  is  able  to  distort  the  field  to  such  an  extent 
that  there  is  no  commutating  flux.  The  absolute  limit  to  field 
weakening  is  complete  open  circuit  in  the  shunt  field.  If  this 
accidentally  occurs,  the  machine  will  tend  to  speed  up  to  a  dan- 
gerous value,  the  high  speed  being  necessary  to  generate  the  proper 
value  of  C.E.M.F.,  since  only  residual  flux  is  present.  Mean- 
while it  also  draws  an  excessive  value  of  current  from  the  line. 

When  the  brushes  are  shifted  in  a  motor  not  equipped  with 
commutating  poles,  as  in  the  generator,  armature  reaction  is 
intensified.  For  a  backward  shift,  due  to  the  creation  of  more 
back  ampere-turns,  part  of  the  main  field  flux  is  neutralized  and, 
as  above,  the  speed  rises.  Furthermore,  the  effect  of  shifting 
brushes  is  to  decrease  the  effective  number  of  inductors  upon  the 
armature.  In  Figs.  16  and  17  is  shown  a  motor  armature  in 


THE    SHUNT   MOTOR 


37 


which  the  inductors  marked  with  a  cross  carry  current  away 
from  the  observer,  those  marked  with  a  dot  toward  the  observer. 
We  have  seen  that  when  the  conductors  upon  the  armature 
of  a  motor  revolve  in  the  magnetic  field,  there  is  generated  in 
each  an  E.M.F.  which  is  in  the  opposite  direction  to  that  im- 
pressed upon  the  armature.  This  generated  E.M.F.  was  called 
the  counter  E.M.F.  Considering  only  the  C.E.M.Fs.  in  the  case 
of  the  conductors  to  the  left  of  the  line  aa  in  Fig.  16,  the  ends 
toward  the  observer  are  positive  with  respect  to  the  ends  away 
from  the  observer.  In  those  to  the  right  of  aa',  the  observer  is 
looking  at  the  negative  end  of  the  conductor.  In  an  armature 


FIG.  17 


the  individual  conductors  are  always  so  connected  that  the  posi- 
tive end  of  one  of  the  right-hand  conductors  is  joined  to  the 
negative  end  of  a  conductor  on  the  left  of  aa'.  Thus  all  of  the 
conductors  are  joined  to  add  their  individual  C.E.M.Fs. 

If  the  brushes  are  now  shifted  backwards  to  the  position  bb',  as 
shown  in  Fig.  17,  the  current  in  conductors  7,  8,  15  and  16  is 
reversed,  but  as  these  conductors  are  still  cutting  the  flux  as 
before,  the  upper  ends  of  7  and  8  will  still  be  negative  and  those 
of  15  and  1 6  positive  with  respect  to  the  lower  ends.  Since  the 
upper  end  of  each  conductor  above  bb'  is  joined  to  the  upper  end 
of  some  conductor  below  bb',  then  some  of  the  conductors  are 
connected  in  the  improper  order,  i.e.,  the  positive  ends  of  some 
of  the  conductors  are  joined  to  a  positive  end  of  another.  The 
C.E.M.Fs.  of  some  of  the  conductors  thus  oppose  those  gen- 


38 


TESTING   OF   ELECTRICAL   MACHINERY 


crated  in  others,  and  the  effect  is  equivalent  to  decreasing 
the  effective  number  of  conductors.  Since  e  =  K'cfNn  (where  n  is 
the  effective  number  of  conductors)  it  follows  that  if  e  is  to  remain 
at  its  proper  value,  the  speed  must  increase. 

We  have  seen  before  that  the  effect  of  additional  load  is  to 
cause  the  speed  of  a  shunt  motor  to  decrease.  Increase  of  arma- 
ture current  is  also  accompanied  by  greater  armature  reaction, 
whose  effect,  as  we  have  just  seen,  is  to  decrease  the  effective 
flux  and  thereby  increase  the  speed.  It  might  then  be  argued, 
that  if  armature  reaction  were  made  great  enough,  a  constant 
speed  shunt  motor  might  be  designed.  Whereas  this  is  possible, 
it  is  never  done,  armature  reaction  being  kept  as  low  as  possible  to 
obtain  good  commutation.  Besides,  the  drop  in  speed  in  a  shunt 
motor,  from  no  load  to  full  load,  being  only  a  few  per  cent.,  is  of 
no  great  commercial  importance. 

In  order  to  change  the  direction  of  rotation  in  a  motor  the 
relation  of  armature  current  to  field  flux  must  be  changed.  This 
involves  reversing  either  the  armature  or  the  field  current.  If 
both  are  reversed,  their  relation  remains  the  same  and  the  armature 
continues  to  rotate  in  the  same  direction  as  before. 

Connect  the  shunt  motor  to  be  used  as  in  Fig.  18,  choosing 


Field 


FIG.  i 3 


ammeters  and  variable  rheostats  of  suitable  current  capacity. 
The  terminals  of  the  starting  box  will  generally  be  marked  as  in 
the  diagram  and  they  should  be  connected  as  indicated.  The  start- 
ing box  shown  has  four  terminals,  of  which  two  are  marked 
"line";  one  of  these  will  be  a  large  terminal  (marked  "  a"  in 


THE    SHUNT   MOTOR  39 

the  diagram),  the  other  is  a  small  terminal  (marked  "  b  ").  Many 
starting  boxes  are  provided  with  only  three  terminals,  and  in  this 
case,  line  terminal  b  is  the  one  omitted  and  the  connection  b,  F%, 
is  left  out. 

With  all  of  the  resistance  cut  out  of  both  rheostats,  close 
the  line  switch  and  slowly  move  the  handle  of  the  starting  box 
as  far  as  it  can  go.  If  the  starting  box  is  properly  connected  the 
handle  will  remain  in  this  position. 

(1)  With  the  machine  running  free,  adjust  the  brushes  by  shift- 
ing backward  and  forward,  to  obtain  the  minimum  speed ;   note 
that  at  this  point  the  sparking  is  a  minimum.     It  is  known  as 
the  no-load  neutral  point. 

(2)  Shift  the  brushes  forward  a  small  amount,  note  speed  and 
sparking   and   read    the   meters.     Again   move   them  forward 
an  equal  amount  and  repeat.     Continue  this  until  either  the 
speed  rises  25  per  cent  or  the  sparking  becomes  bad.     Repeat, 
moving  the  brushes  backward. 

NOTE  :  If  the  machine  assigned  is  provided  with  commutat- 
ing  poles,  perform  i  and  2  upon  another  machine. 

(3)  With  the  brushes  on  the  neutral  point  and  no  load  on  the 
motor,  insert  resistance  into  the  shunt  field  circuit  by  means  of 
VRf.     Do  not  raise  the  speed  more  than  25  per  cent  above  the 
normal  value.     Carefully  determine  speeds  and  read  all  meters. 
Take  from  8  to  10  readings,  recording  in  a  log  as  in  Table  V. 
Put  load  on  the  motor  by  means  of  a  brake  and  repeat,  holding 
the  armature  current  constant  at  one-half  rated  value  by  adjust- 
ing the  brake  tension. 

TABLE  V 


Armature 
Volts. 

Armature 
Amperes. 

Field 
Amperes. 

Speed. 

Armature 
IR  Drop. 

C.E.M.F. 

(4)  With  no  load  on  the  motor  insert  resistance  into  the 
armature  circuit  by  means  of  VRA  ( VRF  should  be  all  cut  out) , 
again  noting  speed  variations  and  reading  all  meters.  Repeat, 


4:0  TESTING   OF   ELECTRICAL   MACHINERY 

holding  the  armature  current  constant  at  one-half  rated  value,  by 
means  of  a  brake. 

(5)  Investigate  the  methods  of  reversing  the  direction  of 
rotation  of  the  armature. 

(b)  The  rating  of  a  motor  is  usually  given  in  horse-power,  and 
is  the  actual  or  available  horse-power  at  the  motor  pulley.  To 
determine  the  horse-power  output  of  a  motor,  we  must  know  the 
pull  F,  which  the  motor  is  capable  of  exerting  at  the  periphery  of 
the  pulley,  and  also  L,  the  radius  of  the  pulley.  In  one  revolu- 
tion the  point  of  application  moves  a  distance  2irL  with  respect 
to  the  pulley,  and  in  one  minute,  if  N  expresses  the  R.P.M.,  it 
moves  a  distance  2irLN.  The  work  done  in  foot-pounds  per 
minute  is  then  2-n-FLN,  and  since  33,000  foot-pounds  per  minute 
is  equivalent  to  one  horse-power  we  have 

' 


33,000       5250 
where  L  is  expressed  in  feet,  and  T  is  the  torque  in  pound-feet. 

The  efficiency  of  a  motor  is  the  ratio  of  the  power  which  the 
motor  gives  out  in  mechanical  form,  to  that  which  it  receives  in 
electrical  form.  The  most  direct  way  of  getting  the  efficiency 
is  to  measure  both;  to  get  the  mechanical  output  a  brake  is  most 
commonly  used. 

The  principle  of  a  mechanical  brake  is  that  mechanical  energy 
is  transformed  into  heat  by  friction.  One  of  the  simplest  forms 
of  brakes  is  one  in  which  a  leather  or  canvas  belt  is  wrapped  partly 
around  the  pulley,  as  shown  in  Fig.  19.  The  belt  is  suspended 
from  two  spring  balances,  one  of  which  is  suspended  from  a  hook 
in  some  form  of  frame,  the  other  being  hung  from  a  threaded  rod 
which  passes  through  the  frame  and  engages  a  handwheel.  To 
adjust  the  brake,  it  is  first  necessary  to  allow  the  belt  to  hang 
loose,  when,  due  to  the  weight  of  the  belt  there  will  be  a  small 
pull  upon  both  balances.  This  must  be  noted  and  subtracted 
from  all  subsequent  readings.  Any  load  can  now  be  put  upon 
the  motor  by  increasing  the  tension  of  the  belt,  which  causes 
greater  friction.  In  order  that  the  pulley  shall  not  get  too  hot,  it 
is  generally  of  the  water-cooled  type,  i.e.,  built  to  hold  water  in 


THE    SHUNT    MOTOR 


41 


the  inside  of  its  rim.     The  pull  exerted  by  the  motor  is  the 
difference  between  the  net  readings  of   the  spring  balances. 

(A  steelyard  may  be 
substituted  for  bal- 
ance Si.) 

Another  form  of 
brake  which  is  very 
commonly  used  is 
known  as  the  Prony 
brake.  A  good  form 
of  it,  as  shown  in 
Fig.  20,  consists  of  a 
beam  of  wood  hol- 
lowed out  at  one  end 
to  fit  the  pulley. 
Around  the  pulley  are 
placed  two  thin  iron 
straps  on  the  inside 
of  which  are  placed 

small  blocks  of  wood.  One  end  of  each  strap  is  fixed  and  the 
other  ends  are  attached  to  a  threaded  rod  which  passes  through 
the  beam  and  can  be  moved  up  or  down  by  means  of  a  9 
nut  or  threaded  handle.  The  tension  of  the  brake  can 
thus  be  varied  at  will. 

The  brake  described  above  has  the  effect  of  increas- 
ing the  radius 
of  the  pulley, 
as  the  force  is 
measured  in  a 
direction  per- 
pendicular to 
a  line  passing 
through  the 
center  of  the 
shaft  or  pul- 
ley. In  the  FIG.  20 


42 


TESTING   OF   ELECTRICAL   MACHINERY 


form  of  brake  described  above  it  is  rather  difficult  to  do  this,  as  the 
line  from  the  center  of  the  pulley  to  the  point  of  application  of  the 
force  is  to  a  certain  extent  imaginary.  Considering  Fig.  21,  we 
have  that  the  torque  is  equal  to  the  product  of  F  and  L.  We 
have,  however,  that 


or    L  =  — 

cos  0 


Now  F=F'  cos  0.     It  follows  then  that 


~~^F'  cos 


If  the  arm  of  the  brake  is  held  horizontally,  the  torque  of  the 


FIG.  21 

motor  will  be  given  by  the  length  of  the  lever  arm  multiplied  by 
the  pull  indicated  by  a  spring  balance  pulling  in  a  direction  perpen- 
dicular to  the  brake  arm.  In  order  to  determine  the  pull  exerted  by 
the  brake  itself,  clamp  it  fast  to  the  pulley  and  slowly  raise  and  lower 
the  brake,  taking  readings  upon  the  spring  balance.  The  average 
of  these  two  readings  gives  the  pull  due  to  gravity  of  the  brake. 
Remember  to  hold  the  spring  balance  perpendicularly  to  the  brake 
arm  when  raising  or  lowering  it. 

In  Fig  22  are  shown  the  characteristic  curves  of  a  shunt 
motor.  It  will  be  seen  that  the  efficiency,  at  first,  rises  very 
rapidly  with  increasing  H.P.  output,  then  slowly  bends  over, 


THE   SHUNT   MOTOR 


43 


becomes  horizontal  and  falls  on  overload.  This  is  due  to  the 
fact  that  the  losses  of  the  machine  are  at  the  start  nearly  constant, 
but  increase  more  and  more  rapidly  until  on  overload  they  increase 
faster  than  the  output.  This  is  principally  due  to  the  armature 
copper  loss,  which  varies  as  the  square  of  the  armature  current. 
The  same  factors  cause  the  curve  plotted  between  H.P.  output 
and  amperes  input,  to  become  concave  upward  on  overload. 

To  determine  the  commercial  efficiency  of  the  motor  to  be 
tested,  connect  it  up  as  in  part  (a),  omitting  the  two  variable  rheo- 
stats. Determine  the  full  load  armature  current  and  operate  the 
motor,  first  with  no  load  upon  it  and  then  with  f ,  J,  f ,  ^,  },  £  and  | 
full  load  armature  current.  Read  all  three  meters  and  the  two 
spring  balances  and  determine  the  speed  for  each  setting.  Record 
readings  in  a  log  as  shown  in  Table  VI. 


H.P.  Output 

-IG.    22 


Torque 


LIcaGure  tlie  armature  circuit  resistance  for  the  same  range 
of  currents  as  used  above.  Use  the  values  here  determined  in 
the  calculations  for  both  parts  (a)  and  (b). 

Curves. — (a)  Plot  a  curve  between  field  current  and  speed 
from  the  results  of  run  3.  Upon  the  same  sheet  of  cross-section 
paper  plot  a  curve  between  armature  E.M.F.  and  speed  from  the 
results  of  run  4.  Plot  speed  as  abscissa  in  both  cases. 


44 


TESTING   OF   ELECTRICAL   MACHINERY 


(b)  Upon  a  second  sheet  of  cross-section  paper  plot  a  set  of 
curves  as  indicated  in  Fig.  22.  Plot  a  curve  between  armature 
resistance  and  armature  current  upon  a  separate  sheet. 

TABLE   VI 


A 
Volts. 

B 

Armature 
Amperes. 

C 

Field 
Amperes. 

D 

Total 
Amperes. 

E 
C.E.M.F. 

F 

Spring 
Balance  Si 

G 

Spring 
Balance  82 

B  +  C 

Initial 
Reading  = 

Initial 
Reading  = 

H 

Net  Pull. 

I 
Torque. 

J 

Watts 
Input 

K 
H.P. 

Input. 

L 

H.P. 
Output. 

M 
Per  cent 
Efficiency. 

N 
R.P.M. 

AXD 

Note. — When  using  Prony  Brake  omit  column  G. 

Conclusions. — What  are  the  principles  upon  which  motor 
operation  depends?  How  can  the  speed  of  a  shunt  motor  be 
increased  above  its  rated  value?  How  can  it  be  lowered  from 
its  rated  value?  Give  the  reasons  for  these  results.  What 
are  the  disadvantages  of  each  method  ?  Why  is  the  curve  between 
field  current  and  speed  concave  upward  ? 

Explain  the  form  of  the  load  characteristics.  Why  does  the 
speed  of  a  shunt  motor  fall  off  slightly  from  no  load  to  full  load  ? 
Why  must  the  armature  resistance  be  made  low?  What  is  the 
speed  regulation,  in  per  cent,  of  the  motor  tested?  What  is 
the  effect,  on  speed  regulation,  of  resistance  in  series  with  the 
armature? 


EXPERIMENT  VI 

The  Motor  Starting  Rheostat.  We  have  seen  that  when 
current  flows  through  an  armature  that  there  is  an  IR  drop,  which 
when  multiplied  by  the  current  gives  PR,  the  rate  of  production 
of  heat.  If  at  ordinary  temperatures  the  armature  is  unable  to 
radiate  this  heat  as  fast  as  it  is  produced,  it  may  rise  to  a  tempera- 
ture of  100°  C.  or  more,  at  which  values  of  temperature  the 
various  kinds  of  insulation  upon  the  armature  will  be  damaged. 
Accordingly  the  current  capacity  of  a  machine  is  taken  as  that 
value,  which,  with  continuous  operation  will  not  cause  the  tempera- 
ture of  the  armature  to  rise  more  than  50°  C.  above  a  room 
temperature  of  40°  C.*  A  machine  is,  however,  usually  capable 
of  carrying  a  large  current  for  short  intervals,  say  150  per  cent  of 
its  full  load  value  for  half  an  hour.  It  might  also  be  possible  for 
a  motor  to  take  an  instantaneous  current  of  many  times  its  rated 
value  without  undue  injury,  but  such  a  current  value  would  be 
beyond  the  range  of  the  protective  apparatus,  such  as  fuses  and  cir- 
cuit breakers.  These  protect  against  comparatively  small  over- 
loads which  if  continuously  applied  would  burn  out  the  machine. 

When  the  armature  of  a  motor  is  at  rest  and  an  E.M.F.  is 
applied  to  its  terminals,  there  is  only  the  resistance  reaction 
to  balance  the  applied  voltage,  so  that  a  current  will  flow  through 
the  armature  according  to  the  relation. 


where  Ia  =  armature  current  ; 

Ea  =  voltage  impressed  across  the  armature; 

Ra=  armature  resistance. 

The  armature  resistance  being  very  low,  it  follows  that  if  the 
value  of  Ea  were  that  of  the  line  upon  which  the  motor  is  intended 
to  operate,  the  armature  current  would  be  excessive. 

*  The  matter  of  temperature  ratings  of  electrical  machinery  is  a  complicated 
one.     See  Standardization  Rules  of  the  A.I.E.E.  and  of  the  Electric  Power  Club. 

45 


46  TESTING    OF  ELECTRICAL  MACHINERY 

The  most  natural  method  of  j  reventing  an  abnormal  current 
is  to  insert  resistance  in  series  with  the  armature,  so  that  we  have 


7- 

J-  n  - 


where  Ia  and  Ra  are  the  same  as  before; 
Et  =  voltage  of  supply  circuit; 
Rx= external  resistance  in  series  with  armature. 

Let  us  leave  this  discussion  for  a  time  and  consider  the 
conditions  which  a  motor  must  fulfill  in  order  to  properly  start  a 
load.  In  most  cases  a  motor  at  starting,  must  exert  its  full  load 
value  of  torque  and  in  many  cases  a  value  greater  than  this.  This 
means  that  the  resisting  torque  of  a  load  during  starting  of  the 
motor,  is  equal  to  or  somewhat  greater  than  when  running.  The 
question  of  acceleration  is  also  of  considerable  importance,  for 
while  desirable  that  the  starting-up  period  be  reasonably  short,  it 
must  not  be  so  short,  as  to  demand  such  a  value  of  torque  which 
might  cause  a  shaft  to  be  bent  or  a  belt  to  slip.  It  is  usually 
found  that  if  a  motor  is  capable  of  exerting  a  torque  of  100  to 
150  per  cent  of  its  full  load  value,  that  the  rate  of  acceleration 
is  of  the  proper  value. 

The  torque  or  turning  effort  of  a  motor  depends  only  upon 
the  current  through  the  armature  and  upon  the  field  flux,  and 
is  entirely  independent  of  speed.  It  follows  from  this  that  the 
value  of  the  field  current  should  be  a  maximum,  as  then  we  can 
obtain  any  value  of  torque  within  the  range  of  the  motor,  with 
minimum  armature  current.  A  shunt  motor  is  nearly  always 
so  operated  (i.e.,  with  its  field  connected  directly  across  the  line) 
and  then  when  full  load  current  flows  through  the  armature 
the  motor  exerts  its  full  load  torque.  If  we  desire  twice  the  full 
load  value  of  torque  we  must  supply  double  the  armature  current. 

Therefore,  to  properly  start  a  shunt  motor  we  must  connect 
its  field  directly  across  the  supply  line,  so  as  to  obtain  maximum 
value  of  flux  and  then  connect  the  armature  to  the  supply  line 
in  series  with  an  external  resistance  of  such  a  value,  that  an 


THE  MOTOR    STARTING    RHEOSTAT 


47 


armature  current  equal  to  ico  to  150  per  cent  of  the  full  load 
rating  of  the  machine  will  flow. 

The  resistance  in  series  with  the  armature  also  serves  another 
purpose,  namely,  that  of  reducing  the  voltage  applied  to  the 
armature.  The  effect  of  this  (as  we  have  seen  in  the  previous 
experiment)  is  to  reduce  the  speed.  As  soon  then  as  the  armature, 
with  resistance  in  series  is  connected  to  the  line,  the  current 
lises  to  100  to  150  per  cent  of  the  full  load  value.  This 
reacting  with  the  field  flux  produces  a  torque  which  starts  the 
armature  rotating.  As  soon  as  it  starts  to  rotate,  a  C.E.M.F. 


123456 
.Points  vn  Starting  Box 

flG.    23 

is  generated,  which  in  turn  reduces  the  current  to  a  smaller 
value  and  the  motor  continues  to  rotate  at  a  speed  somewhat 
below  its  rated  value.  If  now  a  little  of  the  starting  resistance 
is  taken  out,  there  will  be  another  increase  of  the  current,  more 
torque  will  be  exerted  and  the  speed  will  rise  to  a  higher  value. 
The  sudden  increase  in  current  each  time  the  starting  resistance 
is  reduced,  is  occasioned  by  the  fact  that  the  voltage  across  the 
armature  is  suddenly  increased,  whereas  the  C.E.M.F.  at  that 
instant  is  at  the  value  fixed  by  previous  conditions.  Finally  when 
all  of  the  starting  resistance  has  been  taken  out,  the  motor  is 
operating  at  full  speed  directly  upon  the  line.  The  variations 
of  the  armature  current  during  starting  under  full  load  are  sho\\  n 
in  Fig.  23. 

Motor-starting  rheostats  are  made  up  in  many  different  forms; 
a  rather  completely  equipped  form  is  shown  in  Fig.  24.    The 


48 


TESTING   OF   ELECTRICAL  MACHINERY 


starting  handle  is  shown  in  a  position  cutting  out  part  of  the 
resistance.  Current  enters  let  us  say  at  the  terminal  L  and  flows 
around  a  solenoid  B,  through  a  switch-blade  contact  C,  along 
the  movable  arm  D  to  the  movable  arm  E.  The  current  then 
divides,  part  going  through  the  resistances  r%,  r±,  r^  to  the  ter- 
minal A,  which  is  connected  to  the  armature.  The  remainder 
of  the  current  goes  through  the  resistances  ri,  r2,  to  the  terminal 
F,  which  is  connected  to  the  field.  These  currents  unite  at  M> 
flowing  back  to  the  line  on  a  common  conductor. 


FIG.  24 

When  the  motor  is  standing  still,  the  main  switch  is  open 
and  the  arm  E  of  the  starting  rheostat  is  over  to  the  left  against 
the  stop  N,  being  held  there  by  a  spiral  spring  P  at  the  pivot 
of  the  arms.  To  start  the  motor,  the  line  switch  is  first  closed 
and  the  arm  E  slowly  moved  over  to  the  right  against  the  ten- 
sion of  the  spring  P,  until  it  is  on  the  last  notch.  Here  the 
arm  is  held  by  the  attraction  of  an  electromagnet  H  for  the. 
iron  keeper  K  upon  the  arm.  This  device  is  commonly  called 
the  "  no-voltage  "  release.  The  electromagnet  is  energized  by 
current  flowing  from  P  and  through  the  high  resistance  R,  this 
circuit  being  thus  connected  directly  across  the  supply  line.  When 
the  main  switch  is  opened  or  the  line  potential  fails,  the  solenoid 


THE   MOTOR    STARTING   RHEOSTAT 


49 


H  is  demagnetized  and  the  starting  arm  swings  back  to  the 
starting  position,  automatically  protecting  the  motor  for  the  next 
starting  or  from  a  big  inrush  current  if  the  line  potential  were 
again  applied. 

The  other  automatic  device  is  the  overload  release,  which  is 
essentially  a  circuit  breaker.  A  solenoid  B  is  placed  in  series 
with  the  main  circuit  and  when  the  motor  current  momentarily 
exceeds  a  certain  limit,  the  plunger  Q  is  attracted  upward  and 
strikes  a  trigger  T.  This  releases  the  arm  D,  which  swings  in 


FIG.  25 

toward  E,  due  to  the  action  of  the  spring  P.  The  circuit  is  thus 
automatically  opened  at  C.  The  lever  D  cannot  be  reset  without 
moving  the  arm  E  back  beyond  its  starting  position. 

In  a  majority  of  the  starting  rheostats  in  use,  the  overload  device 
is  not  used,  the  line  fuses  being  relied  upon  to  open  the  circuit 
in  case  of  overload.  The  advantage  of  having  the  overload  release, 
is  that  it  is  easily  reset,  whereas  it  takes  time  to  put  in  new  fuses. 

Another  form  of  starting  rheostat  is.  shown  in  Fig.  25,  in 
which  it  will  be  seen  that  the  winding  of  the  no-voltage  release 
is  in  series  with  the  field,  instead  of  being  connected  directly 
across  the  line  as  in  Fig.  24.  This  is  an  excellent  arrangement, 
as  it  guards  against  accidental  opening  of  the  field.  There  is 
the  disadvantage  that  a  certain  size  starting  box  cannot  be  used 


50  TESTING   OF  ELECTRICAL  MACHINERY 

v 

on  all  motors  of  the  same  size  and  voltage,  inasmuch  as  the  value 
of  the  field  current  may  not  be  the  same  for  the  various  different 
motors.  The  manufacturer  of  the  motor  can  decide  upon  any 
value  of  field  current  he  chooses,  as  field  excitation  is  only  a 
matter  of  ampere-turns.  Thus  in  one  motor  the  field  current 
rriay  not  be  high  enough  to  give  the  electromagnet  sufficient 
excitation  to  hold  the  starting  arm,  in  another  it  may  be  so  large 
as  to  cause  the  winding  of  the  magnet  to  heat. 

The  type  of  overload  release  shown  on  Fig.  25  is  also  dif- 
ferent from  that  in  Fig.  24.  In  this  case  the  arm  P'  is  provided 
with  a  small  piece  of  copper  strip  S'f  bent  as  shown.  When  P' 
is  attracted  due  to  an  overload  current  flowing  through  the 
solenoid  B',  the  copper  strip  makes  contact  with  two  comple- 
mentary strips  of  copper  and  thus  short  circuits  the  winding 
of  the  no-voltage  release.  As  a  result  the  latter  becomes 
de-energized  and  permits  the  starting  arm  to  swing  back  to  the 
starting  position.  The  trouble  with  this  device,  though  cheaper  to 
manufacture,  is  that  the  auxiliary  contacts  are  liable  to  be  bent  or 
become  dirty  and  thus  inoperative.  The  form  of  overload  release 
shown  in  Fig.  24  is  more  positive  in  its  action  and  more  reliable. 

For  this  experiment,  a  starting-box  is  necessary  which  will 
permit  a  motor  to  be  operated  at  full  load  current  at  the  various 
notches  for  short  intervals  without  dangerous  overheating.  Con- 
siderable energy  is  transformed  into  heat  in  the  box  under  these 
conditions  and  most  types  will  be  injured  if  so  operated.  There 
are,  however,  a  few  excellent  types  upon  the  market  in  which  the 
resistances  are  imbedded  in  sand  with  no  soldered  connections 
and  which  are  absolutely  fireproof. 

Connect  the  motor  to  be  used  in  studying  the  operation  of 
the  starting  box  assigned,  as  in  Figs.  24  or  25.  The  ammeter 
to  be  used  in  the  armature  circuit  must  have  a  range  equal  to 
double  the  full  load  armature  current  of  the  motor.  The  two 
voltmeters  should  have  a  range  a  little  larger  than  the  line  voltage. 
Provide  a  slip  of  paper  for  each  meter  upon  which  the  readings 
can  be  jotted  down,  as  there  will  not  be  time  to  record  them  in 
the  log.  Three  men  are  really  needed  for  rapid  work,  one  to 


THE   MOTOR   STARTING   RHEOSTAT 


manipulate  the  starting  lever  and  read  one  voltmeter  and  the 
field  ammeter,  the  second  to  read  the  remaining  meters  and  the 
third  to  take  speed. 

With  no  load  upon  the  motor  close  the  line  switch  and  put 
the  starting  lever  upon  the  first  notch  of  the  rheostat.  At  the 
same  time  the  maximum  throw  upon  the  armature  ammeter 
must  be  noted  and  immediately  recorded.  As  soon  as  the  motor 
has  reached  steady  speed,  as  determined  by  a  tachometer,  a 
reading  upon  all  the  meters  is  to  be  taken.  Then  the  starting 
lever  is  to  be  moved  to  the  second  notch,  the  maximum  reading 
upon  the  armature  ammeter,  the  speed  and  all  the  steady  values 
are  again  to  be  recorded.  This  is  continued  until  the  running 
position  has  been  reached. 

Then  put  a  brake  upon  the  motor  and  with  the  motor  running 
£t  full  speed  tighten  the  brake  until  the  armature  current  is  about 
So  per  cent  full  load  value.  Then  without  changing  the  tension 
of  the  brake  stop  the  motor  and  proceed  as  before. 

//  is  imperative  that  the  readings  be  taken  rapidly,  else  the  box 
will  heat  excessively. 

After  the  second  run,  determine  the  armature  resistance  for 
several  values  of  current,  covering  the  range  recorded  in  the  test. 

TABLE    VII 


« 

2 

3 

4 

5 

6 

7 

8 

9 

Arma- 
ture 
E.M.F. 
VA 

Box 
E.M.F. 

VB 

Line 
E.M.F. 

Armature  Current. 

Field 
Current. 

C.E.M.F 

Speed. 

Box 
Resist- 
ance. 

Maxi- 
mum. 

Steady. 

I  +  2 

Record  readings  in  a  log  as  in  Table  VII,  using  steady  values 
of  armature  current  to  calculate  C.E.M.F.  and  box  resistance. 


52  TESTING    OF  ELECTRICAL  MACHINERY 

Curves.  Plot  a  curve  between  armature  current  and  notches 
on  starting  box  as  indicated  in  Fig.  23,  for  both  no  load  and  full 
load  runs.  Also  plot  curves  for  both  runs  between  speeds 
(abscissa)  and  C.E.M.F. 

Conclusions.  Why  is  a  starting  rheostat  necessary?  Why  is 
it  essential  to  provide  maximum  value  of  field  current  at  the 
first  point?  Why  should  a  motor  be  started  slowly?  How  do 
armature  and  field  currents  vary  during  starting?  Why  cannot 
the  usual  type  of  starting  box  be  used  for  continuous  operation 
with  the  lever  upon  any  of  the  starting  notches?  What  determines 
the  steady  speed  and  armature  current  for  each  notch  on  the 
starting  rheostat? 


EXPERIMENT   VII 


Efficiency  of  a  Shunt  Motor  by  the  Stray  Power  Method. 
The  electrical  input  to  a  motor  is  not  all  converted  into  mechan- 
ical energy,  some  of  it  going  to  waste  in  various  ways.  Accord- 
ingly we  may  write  that 

Input  =  Output  -{-Losses,     .....     (14) 
and  also 

Output     Input—  Losses 

-=—  d~) 

input  input 


Commercial  Efficiency  = 


The  losses  in  any  motor  or  generator  can  be  divided  as  follows: 

|  Copper  loss  in  armature 

{  Copper  loss  in  fields 

r  Bearing  Friction 
j  Air  friction  or  windage 
Brush  friction 


Losses 


Stray  Power 


Friction  or 
Mechanical  losses 


Iron  or        f  Hysteresis  loss 
Core  losses  [  Eddy  current  loss. 


Upon  consideration  of  the  copper  losses,  it  is  evident  that  if  we 
know  the  resistance  of  the  armature  circuit  we  can  immediately 
compute  the  I2R  loss  for  any  value  of  current.  This  is  true  for  a 
current  actually  measured  or  assumed  and  the  same  applies  to 
the  fields. 

The  friction  losses  will  vary  directly  writh  the  speed  within 
the  limits  between  which  the  machine  is  operated.  The  hys- 
teresis loss,  however,  varies  directly  as  the  speed  and  the  1.6 
power  of  the  magnetic  density  in  the  armature  iron.  The  eddy 
current  loss  varies  as  the  square  of  both  the  speed  and  the  mag- 
netic induction. 

In  the  shunt  motor  as  commercially  operated  t\vo  factors 
are  considered  constant,  namely,  the  impressed  E.M.F.  and  the 

53 


54  TESTING  OF  ELECTRICAL  MACHINERY 

shunt  field  current.  Let  us,  for  the  moment,  neglect  the  effect  of 
armature  reaction  upon  the  flux  and  therefore  consider  the  latter  as 
being  constant  under  all  conditions  of  load. 

When  the  load  upon  the  motor  was  increased,  we  found  that 
there  was  a  decrease  in  C.E.M.F.  and  a  slight  drop  in  speed. 
From  the  equation  e  =  K$N  it  follows  that  if  the  flux  is  consid- 
ered constant,  the  C.E.M.F.  is  directly  proportional  to  the  speed. 

The  equation  for  the  armature  current  of  a  shunt  motor  is, 
as  we  have  seen, 

T  _ 


a        R    ' 

so  that,  knowing  the  resistance  of  the*  armature,  we  are  able  to 
calculate  the  value  of  the  C.E.M.F.  for  any  value  of  armature 
current.  Nor  does  it  make  any  difference  whether  this  value 
of  armature  current  is  one  actually  obtained  in  practice  or 
one  assumed.  Then  if  we  know  the  value  of  the  speed  and 
C.E.M.F.  at  some  value  of  armature  current,  we  can  calculate 
the  C.E.M.F.  at  any  other  armature  current  and  by  proportion 
obtain  the  speed.  That  is,  we  predict  the  speed  at  which  the 
motor  would  run  if  it  had  a  certain  value  of  armature  current. 
We  can  thus  forecast  the  speed  load  curve  of  a  shunt  motor  pro- 
vided that  we  know  the  armature  resistance  and  the  speed  at 
some  one  value  of  armature  current. 

Consider  the:i  that  we  know  the  -idlues  cf  armature  and  field 
currents  and  speed  when  the  motor  is  running  free,  with  rated  volt- 
age applied  to  both  cr:nature  and  field.  From  the  name-plate  of 
the  motor  we  can  find  what  the  manufacturer  intended  the  full 
load  armature  current  should  be.  From  this  full-load  value 
of  the  armature  current,  we  can  calculate  the  value  of  the  full 
load  C.E.M.F.  and  by  proportion  obtain  the  full  load  speed 
of  the  machine,  using  the  relation 

C.E.M.F.  at  no  load  :  C.E.M.F.  at  full  load  = 

Speed  at  no  load  :    Speed  at  full  load 

The  full  load  speed  will  of  course  be  lower  than  the  no  load  speed. 


EFFICIENCY   BY   THE    STRAY   POWER   METHOD  55 

Knowing  the  full  load  armature  current  we  also  know1  the 
complete  power  input,  that  being  the  sum  of  the  armature  and 
field  currents  multiplied  by  the  terminal  E.M.F.  As  we  know 
the  value  of  armature  and  field  resistances,  we  can  readily  deter- 
mine what  the  armature  and  field  copper  losses  will  be.  So  that 
the  only  other  losses  that  so  far  we  do  not  know,  are  the  stray 
power  losses. 

When  a  motor  is  operating  at  no  load,  that  is,  running  free, 
the  entire  input  is  used  in  overcoming  losses  in  the  machine  itself. 
If  the  known  copper  losses  be  subtracted  from  the  input,  we 
have  left  the  stray  power  loss  at  no  load,  at  a  particular  speed 
and  the  rated  value  of  field  current.  Similarly  we  could  determine 
the  stray  power  loss  at  any  other  speed  and  field  current. 

Suppose  we  knew  the  full  load  speed  of  the  machine  and  were 
to  operate  the  motor  with  no  load  upon  it,  at  this  value  (which 
is  of  course  lower  than  the  no  load  value)  and  at  rated  value  of 
field  current.  We  could  then  calculate  the  stray  power  loss  of  the 
machine,  when  running  at  its  full  load  speed  and  rated  field  cur- 
rent but  with  no  load  upon  it. 

The  effect  of  armature  reaction,  proportional  to  load  current 
and  brush  position,  is  to  modify  somewhat  the  stray  power  loss. 
If  we  neglect  this  effect,  however,  it  is  possible  to  obtain  full 
load  stray  power  or  stray  power  at  any  fractional  load,  without 
actually  loading  the  motor.  If,  e.g.,  the  full  load  speed  is  known, 
then  full  load  stray  power  may  be  obtained  with  the  machine 
running  free  by  measuring  armature  input,  with  normal  field  and 
sufficient  impressed  E.M.F.  to  give  full  load  speed.  Stray  power 
loss  is  then  obtained  by  subtracting  the  armature  copper  loss 
from  the  armature  input.  Then  knowing  all  the  losses  and  the 
total  input,  we  can  readily  determine  the  commercial  efficiency. 

It  is,  however,  necessary  that  we  consider  what  effect  load 
does  have  upon  the  value  of  the  stray  power  losses.  As  soon 
as  armature  current  flows  we  have  armature  reaction,  the  effect 
of  which  is  generally  to  weaken  the  main  field.  This  tends  to 
raise  the  speed,  of  the  motor,  or  in  other  words  the  actual  speed 
load  curve  is  slightly  higher  than  the  predicted  curve.  As  a 


56 


TESTING   OF   ELECTRICAL  MACHINERY 


result  of  these  conditions  the  value  of  stray  power  will  be  slightly 
different  from  those  determined  with  no  load  upon  the  machine. 
However,  the  difference  is  small  and  inasmuch  as  stray  power 
itself  is  a  small  per  cent  of  the  total  input,  the  error  is  negligible. 
The  advantages  of  the  method,  however,  more  than  counter- 
balance this,  for  being  a  prediction  method  there  is  a  great  saving 
in  power.  Waste  of  power  is  unavoidable  in  a  brake  test  and 
even  at  times  prohibitive,  as  the  necessary  power  may  not  be 
available.  Then  again  the  losses  are  measured  by  means  of 
electrical  instruments  which  are  very  accurate,  very  much  more 
so  than  the  forms  of  brakes  used,  which  are  also  difficult  to  operate. 


FIG.  26 

The  method  is  therefore  preferable  in  most  cases,  particularly 
where  the  division  of  the  losses  into  separate  components  is 
desired. 

Before  operating  the  motor  to  be  tested  first  detemine  the 
resistance  of  the  armature  circuit  for  8  values  of  current  between 
a  very  low  value  (2  or  3  per  cent  of  rated  value)  and  150  per 
cent  full  load  current,  and  plot  the  values  as  a  curve. 

Connect  the  motor  as  in  Fig.  26,  the  source  of  power  being 
preferably  a  laboratory  generator  whose  terminal  voltage  is  under 
control.  With  both  VRA  and  VRF  both  out,  operate  the  motor 
with  rated  voltage  impressed  across  both  field  and  armature. 
Let  the  machine  operate  for  about  1 5  minutes  and  then  measure 
speed  and  both  field  and  armature  currents  very  carefully. 

Determine  the  full  load   armature  current  of   the   machine 


EFFICIENCY  BY  THE   STRAY  POWER  METHOD 


57 


from  its  name-plate  and  calculate  the  C.E.M.F.  for  25,  37.5, 
50,  75,  100,  125,  and  150  per  cent  full  load  currents.  Then 
calculate  the  C.E.M.F.  when  running  light  and  with  the  speed 
there  found,  determine  by  proportion  the  speeds  corresponding 
to  the  armature  currents  assumed  above. 

Then  operate  the  machine  at  rated  field  current  and  at  the 
calculated  speeds  and  read  all  meters.     A  good  method  of  pro- 


H.P.  Output 

FIG.  27 

cedure  is  to  set  the  value  of  the  supply  line  a  few  volts  above  the 
rated  value  of  the  machine.  Then  insert  resistance  into  the 
field  circuit  by  means  of  VRr  until  rated  value  of  field  current  is 
flowing.  Then  decrease  the  speed  of  the  machine  to  the  desired 
value  by  inserting  resistance  into  the  armature  circuit  by  means 
of  VRA. 

In  the  log,  as  shown  in  Table  VIII,  part  a  is  for  the  operation 
at  no  load  to  determine  the  stray  power  loss  corresponding  to 
each  load  speed.  All  of  the  columns  in  part  b  are  to  be  cal- 
culated or  assumed  except  column  9,  which  is  taken  from  part  a. 


58 


TESTING    OF  ELECTRICAL  MACHINERY 


z* 


13  -*->  *^ 


91 


o  oJ 


pi -a  ^ 


"  W 


-33 


I    3  •' 

IN     I       73^ 

10}   ° 
5^ 


EFFICIENCY   BY   THE    STRAY   POWER   METHOD  59 

Curves.  Plot  the  loss  curves  determined  for  the  machine,  as 
shown  in  Fig.  27;  also  plot  the  curves  of  calculated  efficiency 
and  speed,  using  same  scale  for  abscissa  as  for  the  loss  curves. 
If  this  experiment  is  performed  on  the  same  machine  as  was 
previously  tested  by  brake  (Ex.  5),  plot  the  efficiency  and  speed 
curves  obtained  by  brake  on  same  curve  sheet  as  those  obtained 
by  this  method. 

Conclusions.  What  is  the  assumption  upon  which  the  stray 
power  method  of  determining  the  commercial  efficiency  of  a 
shunt  motor  is  based?  What  are  the  advantages  of  the  method? 
How  do  the  losses  vary  from  no  load  to  full  load?  Why  does 
the  efficiency  at  first  rise  and  then  fall?  How  much  would  the 
full  load  efficiency  as  determined  in  this  test  be  changed,  if  the  no 
load  stray  power  value,  operating  at  rated  voltage  across  the 
armature  and  field  were  used,  instead  of  that  obtained  by 
operating  at  the  calculated  full  load  speed? 


EXPERIMENT  VIII. 

Series  Motor.  In  the  series  motor,  the  field  winding  is  placed 
in  series  with  the  armature  and  carries  the  whole  armature 
current,  and  this  fact  causes  its  characteristics  to  be  entirely 
different  from  those  of  the  shunt  motor. 

The  equation  for  the  current  in  a  series  motor  is  as  follows  : 


in  which  all  of  the  terms  are  the  same  as  before  and  Rse  is  the 
resistance  of  the  series  field. 

As  in  the  case  of  the  shunt  motor,  for  the  series  motor  to 
increase  its  armature  current,  there  must  be  a  decrease  in  the 
C.E.M.F.  In  the  shunt  motor,  where,  except  for  armature  reaction 
the  field  flux  was  constant,  this  was  brought  about  by  a  slight 
decrease  in  speed.  In  the  series  motor  we  have  a  variable  flux, 
for  evidently,  if  the  armature  current  increases,  there  will  be 
an  increase  in  flux. 

It  has  been  shown  that  for  any  motor  e^K^N,  and  that  the 
armature  current  can  only  increase  as  a  result  of  a  decrease 
in  e.  The  increase  in  armature  current  at  the  same  time 
brings  about  an  increase  in  ^>,  so  that  if  the  C.E.M.F.  is  to  fall, 
there  must  be  a  large  decrease  in  speed.  In  the  shunt  motor 
the  flux  is  constant  and  as  load  increases  the  speed  falls  only  enough 
to  cause  a  decrease  in  e.  In  the  series  motor,  in  addition  to  this,  the 
speed  must  fall  enough  to  counteract  the  large  increase  in  flux. 
It  would  appear  then  from  the  equation  e  =  K(j>N  that  inasmuch 
as  e  only  changes  a  small  amount,  that  the  speed  load  curve  of 
a  series  motor  is  nearly  an  equilateral  hyperbola.  This  is  not 
quite  the  case,  due  to  the  fact  that  the  iron  of  the  field  becomes 
saturated.  When  the  point  is  reached  where  an  increase  in 

60 


SERIES   MOTOR 


61 


armature  current  causes  only  a  small  increase  in  flux,  then  the 
speed  does  not  fall  as  much  as  before.  The  speed  load  curve 
of  a  series  motor  is  shown  in  Fig.  28.  It  is  evident  from  the 
curve  that  the  speed  changes  very  materially  in  going  from 
no  load  to  full  load.  A  very  dangerous  condition  is,  however, 
reached  when  there  is  no  load  upon  the  motor,  for  under  these 
conditions  the  machine  takes  very  little  current  from  the  line 


CO 


Speea  Load Curve 

of 
Ser/'es  Motor 


Amperes 
FIG.  28 

and  hence  must  have  a  high  value  of  C.E.M.F.  This  is  accom- 
panied by  a  weak  field  and  hence  a  dangerously  high  speed. 
For  this  reason  a  series  motor  is  always  direct  connected 
to  its  load  either  by  a  coupling  or  through  gears  or  chains,  unless 
a  speed  limit  device  is  attached.  In  the  latter  case,  if  the  motor 
speed  rises  above  a  certain  point  it  is  automatically  disconnected 
from  the  line. 

The  torque  of  any  motor  may  be  expressed  by  the  formula 
T=K(f>Ia  and  in  applying  this  to  the  shunt  motor  we  have  seen 
that  inasmuch  as  <j>  was  nearly  constant,  that  T  varied  almost 


62 


TESTING    OF    ELECTRICAL  MACHINERY 


directly  with  Ia.  In  the  series  motor  0  is,  however,  not  constant, 
but  varies  with  Ia.  Starting  with  light  load  upon  the  motor 
we  have  with  an  increase  of  Ia  an  almost  equal  increase  of  </>. 
The  machine  under  these  conditions  is  operating  upon  the  straight 
portion  of  the  magnetization  curve,  so  that  the  torque  varies 


Amperes   Armature    Current 
FIG.  29 

as  7a2;  at  the  same  time  the  motor  is  operating  at  a  high  speed. 
As  the  armature  current  increases  and  the  saturated  portion 
of  the  magnetization  curve  is  reached,  the  increase  in  flux  is  no 
longer  proportional  to  the  current  increase,  so  that  finally  the 
torque  increases  directly  as  the  armature  current. 

The  fact  that  the  series  motor  exerts  its  maximum  torque 
at  stand-still,  (that  being  the  point  where  the  current  is  greatest 
for  a  given  armature  voltage)  constitutes  one  of  its  greatest  advan- 
tages. In  Fig.  29  are  given  curves  of  torque  and  speed  plotted 


SERIES  MOTOR 


63 


against  amperes  armature  current  for  a  shunt  and  a  series  motor 
of  the  same  full  load  rating.  The  speeds  of  both  motors  are  the 
same  at  light  loads,  but  at  full  load  the  speed  of  the  series  motor 
is  very  much  less  than  that  of  the  shunt  machine. 

Now  since  horse-power  output  is  proportional  to  the  product 
of  torque  and  speed,  i.e.,  H.P.  output  =  KTN,  it  follows  that 
in  the  series  motor,  due  to  its  lower  speed,  the  torque  will  be 
much  greater  at  full  load  than  that  of  the  shunt  motor.  To 
exert  the  same  torque  a  much  larger  shunt  motor  would  be 
necessary. 

The  series  motor  is  thus  particularly  valuable  for  work  where 


FIG.  30 

a  load  is  to  be  frequently  started  and  a  great  starting  torque  is 
desired,  as  in  traction  service,  hoists,  cranes,  etc.  When  the 
required  torque  is  large  the  motor  operates  at  a  low  speed,  but 
where  a  light  torque  is  necessary  it  operates  at  a  high  speed. 

If  the  impressed  voltage  of  the  motor  is  decreased,  the  motor 
speed  will  fall  and  it  will  be  found  that  the  speed  varies  almost 
directly  with  the  impressed  voltage. 

The  resistances  of  the  armature  and  series  field  are  always 
made  low  for  reasons  of  efficiency.  The  question  of  regulation 
is  of  no  importance  as  the  series  motor  has  of  itself  a  widely  vary- 
ing speed  characteristic. 

(1)  Determine  the  resistance  of  the  armature  and  series  field, 
taking  readings  up  to  150  per  cent  full  load  armature  current. 

(2)  Connect  the  motor  as  in  Fig.  30,  choosing  an  ammeter  and 


64 


TESTING   OF   ELECTRICAL  MACHINERY 


a  variable  resistance  with  a  capacity  of  about  double  the  rated 
current  of  the  motor.  Start  the  motor  and  reduce  the  voltage 
across  the  armature  to  one-half  of  its  rated  value.  Then  start- 
ing with  a  very  small  load  (no  load  if  possible)  take  a  reading 
of  current  and  speed;  then  gradually  increase  the  load  by  means 
of  a  brake,  keeping  the  voltage  constant  at  half  rated  value. 
Take  eight  readings  up  to  50  per  cent  overload  current.  If  a 
constant  potential  line  of  one-half  the  rated  motor  voltage  is 
available,  it  serves  the  purpose  better  than  the  use  of  a  variable 
resistance.  Record  readings  as  in  Table  IX. 

TABLE    IX 


E.M.F. 
across 
Motor. 

Current. 

Speed. 

C.E.M.F. 

I 

(3)  Impressing  full  voltage  upon  the  motor,  adjust  the  load 
until  the  motor  takes  about  §  of  its  rated  current.     Read  speed  and 
voltage.     Then  decrease  the  voltage  across  the  armature  by  about 
10  per  cent  and  adjust  the  current  to  the  same  value  as  before  by 
means  of  the  brake  and   take  a  set  of  readings.     Continue  to 
decrease  the  armature  voltage,  keeping  the  armature  current  con- 
stant until  6  or  8  readings  have  been  taken. 

(4)  With  rated  voltage  impressed  upon  the  motor  make  a 
complete  brake  test  up  to  50  per  cent  overload  current.     If  the 
motor  is  equipped  with  a  speed  limit  device,  allow  the  motor  to 
run  free  and  determine  the  speed,  current,  and  voltage,  just  at  the 
instant  the  speed  limit  device  disconnects  the  motor  from  the 
line.     If  no  speed  limit  device  is  provided,  adjust  the  brake  until 
the  motor  operates  at  300  per  cent  of  its  rated  full  load  speed. 
Do  not  permit  it  to  rotate  faster  than  this  value. 

Record  readings  in  a  form  similar  to  Table  VI. 

Curves. — Plot  a  curve  between  current  and  the  sum  of  the 
series  field  and  armature  resistances.  Upon  a  second  sheet,  plot  a 
curve  between  speed  as  ordinate  and  armature  current  from  the 


SERIES   MOTOR  65 

results  of  run  2,  and  a  curve  between  speed  and  impressed  voltage 
from  the  results  of  run  3.  Upon  the  same  sheet,  from  the  readings 
of  run  4,  plot  a  speed-current  curve,  a  torque-current  and  an 
efficiency-current  curve,  the  current  being  plotted  as  absicssa 
in  each  case. 

Conclusions.  Explain  why  the  speed  of  a  series  motor  varies 
more  widely  than  that  of  a  shunt  motor  and  why  a  series  motor  of 
the  same  horse-power  rating  as  a  shunt  motor,  exerts  a  greater  full 
load  torque.  Why  is  the  series  motor  better  adapted  to  frequent 
starting  of  heavy  loads  than  the  shunt  motor?  Why  is  the  torque 
curve  concave  upward  at  the  lower  end  and  straight  at  its  upper 
end?  What  is  the  effect  of  lowering  the  terminal  voltage  of  a 
series  motor  ?  Why  should  a  series  motor  never  be  belt  connected 
to  its  load  ? 


EXPERIMENT  IX 

Current-Torque  Curves  of  Different  Types  of  Motors.  This 
experiment  is  intended  to  show  the  value  of  the  torque  exerted 
by  different  types  of  motors  with  various  values  of  armature  and 
field  currents,  for  purposes  of  comparison. 

We  have  seen  that  when  current  flows  through  the  armature 
windings  of  a  motor  whose  fields  are  excited,  there  is  a  torque 
exerted,  which  tends  to  rotate  the  armature.  The  value  of  the 
torque  depends  upon  the  strength  of  the  field  and  upon  the  value 
of  the  armature  current,  i.e.,  T=K<pI. 

In  the  case  of  the  shunt  motor  we  have  a  constant  field  current 
and  if  the  flux  remains  constant  we  would  expect  that  the  torque 
would  vary  directly  as  the  armature  current  and  that  a  curve 
expressing  their  relation,  would  be  a  straight  line.  In  this  case 
the  torque  per  ampere  (i.e.,  the  value  of  torque  divided  by  the 
current)  would  be  constant.  This  is,  however,  not  quite  the  case, 
inasmuch  as  armature  reaction  distorts  and  reduces  the  flux  so 
that  the  torque  per  ampere  decreases  with  the  higher  values  of 
armature  current.  For  the  same  armature  current  and  a  varying 
field  current,  we  do  not  get  a  straight  line  relation  unless  we  are 
operating  upon  the  straight  portion  of  the  magnetization  curve. 
As  the  saturated  portion  of  the  curve  is  reached  the  torque  does 
not  increase  as  rapidly  for  a  given  increase  of  field  current. 

If  to  the  shunt  motor,  a  series  winding  is  added  and  so  con- 
nected that  its  M.M.F.  is  added  to  that  of  the  shunt  field,  we  get 
a  cumulative  compound  motor.  The  shunt  field  current  in  this 
case  again  remains  constant  and  the  series  field  current  increases^ 
being  the  same  as  the  armature  current.  The  result  is  an 
increasing  flux  as  the  armature  current  increases,  so  that  the 
torque  per  ampere  is  greater  than  when  only  a  shunt  winding  is 
used.  If  the  machine  is  very  much  compounded,  that  is,  if  the 
series  turns  furnish  a  large  percentage  of  the  M.M.F.  at  full 

66 


CURRENT-TORQUE   CURVES  67 

load,  the  curve  between  torque  and  current  will  be  slightly  con- 
cave upward. 

In  the  case  of  the  series  motor,  we  of  course  have  the  same 
current  flowing  through  both  field  and  armature.  When  oper- 
ating upon  the  straight  portion  of  the  magnetization  curve,  the 
flux  is  proportional  to  the  current,  so  that  the  torque  varies  as 
the  current  squared : 

T=K<f>I=KI2. 

Under  these  conditions  the  curve  between  torque  and  current 
is  concave  upward.  As  the  field  approaches  saturation  the 
curve  gradually  straightens  out  until  at  the  end  it  becomes  straight. 

We  are  now  in  a  position  to  compare  the  behavior  of  the 
shunt  and  series  motor.  Let  us  assume  that  we  have  a  shunt 
and  a  series  motor,  each  of  about  5  horse-power  and  both  having 
the  same  full  load  efficiency  and  therefore  about  the  same  full 
load  armature  current.  Let  us  further  assume  that  the  shunt 
motor  has  a  full  load  speed  of  about  1200  R.  P.  M.  and  that 
the  series  motor  has  the  same  speed  at  its  lightest  permissible 
load;  the  full  load  speed  of  the  latter  would  then  be  about  500 
or  600  R.  P.  M.  The  light  load  speeds  of  the  two  motors  would 
thus  be  the  same,  which  fact  is  shown  in  Fig.  29.  Since  output 
is  proportional  to  the  product  of  speed  and  torque,  the  torque 
exerted  by  both  motors  is  about  the  same  at  light  load.  At 
full  load,  however,  the  series  motor  must  exert  double  the  torque 
of  the  shunt  motor,  as  its  speed  is  about  half  that  of  the  latter. 
It  follows  that  the  pole  pieces  and  armature  of  the  series  motor 
must  be  hrger  than  those  of  the  shunt  motor  to  provide  and 
accommodate  the  extra  flux,  so  that  the  series  motor  is  generally 
larger  than  the  equivalent  shunt  motor. 

This  again  brings  out  the  great  advantage  of  the  series  motor, 
namely,  that  it  can  exert  a  much  greater  torque  at  higher  loads 
Sian  the  shunt  motor  for  the  same  value  of  armature  current. 

The  compound  motor,  as  we  have  seen,  increases  its  torque 
somewhat  with  load,  with  a  greater  decrease  in  speed  than  the 
shunt  motor,  but  not  as  much  as  in  the  case  of  the  series  motor. 


68  TESTING    OF   ELECTRICAL   MACHINERY 

It  is  very  valuable  for  running  certain  types  of  machine  tools,  as 
punch  presses,  power  shears,  etc.,  which  are  always  provided  with 
a  heavy  flywheel,  the  energy  necessary  for  the  actual  operation 
of  the  tool,  that  is,  say,  shearing  metal,  etc.,  coming  from  the  energy 
stored  in  the  flywheel.  The  motor  slowing  up,  immediately 
develops  a  strong  torque  and  accelerates  the  flywheel.  It  is 
evident  that  the  compound  motor  is  able  to  accelerate  the  fly- 
wheel and  bring  it  up  to  speed  in  a  shorter  interval  of  time  than  the 
shunt  motor.  There  is  also  the  great  advantage  that  a  relatively 
small  motor  is  necessary  to  operate  such  machines  provided  with  a 
heavy  flywheel,  since  the  time  the  machine  is  doing  work  is  only 
a  small  per  cent  of  the  time  it  is  running.  In  the  time  when  the 
machine  is  running  free  the  motor  is  able  to  store  energy  in  the 
flywheel. 

In  order  to  determine  the  torque  exerted  by  a  motor  when  stand- 
ing still,  some  form  of  Prony  brake  is  clamped  fast  upon  its  pulley, 
as  is  shown  in  Fig.  20.  When  current  flows  through  the  armature 
and  fields,  let  us  suppose  that  the  torque  is  so  exerted  as  to  tend 
to  revolve  the  armature  in  a  clockwise  direction.  If  the  brake 
arm  be  slowly  moved  upward  against  the  torque  of  the  motor, 
we  are  then,  first,  lifting  the  brake;  second,  overcoming  the  fric- 
tion of  the  bearings;  and  third,  overcoming  the  torque  of  the  motor. 
This  can  be  written  as  an  equation  as  follows: 

Wu»XL=T+w+f, 

where  TFup  =  the  reading  upon  the  spring  balance  when  the 
brake  moves  the  pulley  against  the  torque  of  the 
armature ; 

T= torque  exerted  by  armature; 
w  =  torque  due  to  weight  of  braK.; 
/=  torque  due  to  bearing  friction; 
L  =  length  of  brake  arm. 

Where  the  brake  is  allowed  to  descend,  the  motor  torque  and 
the  weight  of  the  brake  overcome  the  friction  so  that 

WdownXL=T+w-f', 


CURRENT -TORQUE  CURVES 


69 


adding  the  two  equations  we  have 

/TF«« 


which  eliminates  friction.  To  determine  w,  the  weight  of  the 
brake,  open  the  current  in  both  armature  and  field  and  slowly 
raise  and  lower  the  brake,  taking  readings  on  the  spring  balance. 
The  average  of  these  two  readings  gives  the  pull  due  to  gravity 
of  the  brake  arm  itself,  which  we  have  called  w. 

The  compound  motor  to  be  used  should  be  connected  as  in 
Fig.  31,  the  shunt  field  being  separately  excited  by  the  poten- 


FlG.3I 

tiometcr  method  and  the  series  winding  open.  The  machine 
is  thus  in  reality  a  shunt  motor.  By  means  of  the  variable 
resistance,  set  the  value  of  shunt  field  current  a  trifle  lower  than 
one-half  its  rated  value  and  then  vary  the  armature  current  from 
zero  up  to  150  per  cent  full  load  value  in  about  8  steps,  by  vary- 
ing the  number  of  lamps  burning  in  the  lamp  board,  taking 
torque  and  current  readings  as  previously  noted.  Keep  the 
shunt  field  current  constant  throughout.  Then  raise  the  shunt 
field  current  to  a  value  double  that  used  in  the  first  run  and  pro- 
ceed as  before.  For  a  third  run,  set  the  armature  current  at 
its  full  load  value  and  keep  it  constant.  Then  vary  the  shunt 
field  current  from  zero  to  its  rated  value.  Take  readings  as 
before.  For  a  fourth  run  connect  the  series  field  in  series  with 
the  armature  as  in  Fig.  32,  making  the  machine  a  cumulative 
compound  motor.  Vary  tlu  current  through  series  field  and 


70 


TESTING   OF  ELECTRICAL   MACHINERY 


armature  as  before  with  the  shunt  field  current  as  in  second 
run.     Take  the  same  readings  as  before. 

To  determine  the  curves  for  the  series  motor  it  is  preferable 
to  use  a  machine  of  the  same  voltage  and  H.P.  output  as  the  com- 
pound motor  already  tested,  in  order  that  proper  comparisons  can 
be  made.  Pass  the  same  current  through  both  armature  and  field 
as  shown  in  Fig.  33. 


FIG.  32 

Curves.  —  For  runs  i,  2,  4,  and  5  plot  curves  between  armature 
current  as  abscissa  and  torque  and  torque  per  ampere  armature 
current  as  ordinates.  For  run  3  use  field  current  as  abscissa. 

Plot  corresponding  curves 
on  the  same  sheet  of  cross- 
section  paper. 

Conclusions.  Why  are 
the  current-torque  curves 
for  the  shunt  mo  tor  straight 
lines?  Why  is  it  concave 
upward  at  the  start  for  the 
series  motor?  Why  does 
torque  per  ampere  decrease 
in  the  shunt  motor  and  not 

on  the  compound  and  series  motors?  From  the  data  obtained 
compare  the  action  of  the  three  types  of  motors.  In  run  4, 
what  tests  can  be  made  to  determine  whether  the  series  field 
is  properly  connected  to  make  the  machine  a  cumulative  com- 
pound motor? 


FIG.  33 


EXPERIMENT  X 

Parallel  Operation  of  Shunt  Generators.  In  many  lighting 
and  power  systems  the  load  which  the  station  is  required  to  carry 
is  much  greater  than  the  capacity  of  the  largest  direct  current 
generator  which  is  manufactured.  It  therefore  becomes  neces- 
sary to  operate  several  generators  on  the  same  feeder  system  to 
supply  the  load.  Even  when  the  load  could  be  carried  by  one 
large  unit,  it  is  many  times  preferable  to  install  two  or  irore 
smaller  units  to  carry  the  load,  because  of  the  increased  security 
against  possible  accident.  If  the  load  is  all  carried  by  one  machine 
and  even  a  minor  accident  happens  to  this  machine,  the  station 
is  obliged  to  shut  down  for  repairs.  If  two  or  more  machines 
are  used  to  supply  load,  one  of  them  may  be  shut  down  for 
repairs  and  the  others,  overloaded  perhaps  for  a  short  time,  will 
satisfactorily  supply  the  station  load.  Besides,  it  is  more  efficient 
to  operate  a  small  machine  at  full  load  than  a  larger  one  at  light 
load. 

There  are  two  possible  schemes  for  connecting  B.C.  machines 
together  so  that  they  supply  power  to  a  common  bus,  in  series  or 
in  parallel. 

If  they  are  connected  in  series,  the  voltage  of  the  line  depends 
upon  how  many  machines  are  operating;  while  this  type  of  ser- 
vice has  been  used  for  very  special  purposes,  it  is  not  feasible  to 
use  such  a  system  for  the  ordinary  purposes  of  furnishing  current 
for  light  and  motors.  For  such  purposes  a  constant  potential 
system  is  required  and  obviously  a  number  of  B.C.  machines 
operating  in  series,  their  number  depending  upon  the  load,  could 
not  satisfy  this  requirement.  Also  the  possible  current  output 
of  such  a  series  system  is  the  rating  of  the  smallest  machine 
used. 

71 


72  TESTING   OF  ELECTRICAL   MACHINERY 

If,  however,  several  machines,  each  of  the  voltage  required  by 
the  system,  are  operated  in  parallel  in  the  same  system,  the  latter 
will  be  essentially  a  constant  potential  one  and  the  possible  cur- 
rent output  of  the  station  depends  only  upon  how  many  machines 
are  connected  to  the  system.  The  current  capacity  of  the  station 
is  equal  to  the  combined  capacity  of  all  the  machines  connected 
to  the  buses. 

Although  shunt  wound  generators  are  not  used  very  much  for 
station  work,  compound  wound  generators  serving  the  purpose 
so  much  better,  it  is  necessary  to  examine  the  parallel  opera- 
tion of  shunt  generators,  so  that  the  more  difficult  case  of  compound 

+  Bus  Bar  a 


-O- 
-O- 


-O- 
-O- 
-O- 
-O- 
-O- 


FIG.  34 


generators  may  be  understood.  (The  operation  of  compound 
generators  in  parallel  will  be  considered  in  the  next  experiment.) 

The  analysis  of  the  question  may  be  most  easily  made  by  sup- 
posing first  that  one  machine  (say  No.  i)  is  already  connected  to  a 
load,  and  that  whatever  manipulation  is  carried  out  with  the  second 
machine  (No.  2),  the  bus-bar  voltage  will  be  assumed  constant. 
This  latter  will  not  generally  be  the  case,  but  makes  the  first 
.analysis  simpler. 

Consider  the  machines  connected  as  in  Fig.  34.     Generator 


PARALLEL   OPERATION    OF    SHUNT    GENERATORS  73 

No.   i  is  supplying  load  and  it  is  desired  to  connect  machine 
No.  2  to  the  bus  bars  and  divide  the  load  between  them. 

Let  E  =  voltage  between  bus  bars; 
Eg=  genera  ted  voltge  of  No.  2; 
Ra  =  armature  resistance  of  No.  2  ; 
/2  =  armature  current  of  No.  2. 

Now  if  the  voltage  Eg  is  exactly  equal  and  opposite  to  E}  it 
is  evident  that  when  P2  and  N2  are  closed  no  current  will  flow 
through  the  armature  circuit  of  No.  2. 

We  have  shown  previously  that 

F  —  F 

LL,        LL,g 

=~~~~ 


which  is  also  written 

E=Eg  +  I2Ra  ........     (17) 

If  when  the  switches  P2  and  N2  are  closed,  Eg=E,  it  is  quite 
evident  from  the  above  equation,  that  no  current  will  flow  through 
the  armature  of  machine  No.  2.  But  if  E9  is  not  equal  to  E  then 
current  will  flow  upon  closing  the  switches,  the  magnitude  of  the 
current  being  determined  by  the  difference  in  E(J  and  E,  and  the 
direction  of  the  current  depending  upon  whether  Eg  is  greater  or 
less  than  E.  If  Ea  is  greater  than  E,  current  will  flow  in  the  same 
direction  as  Eg  tends  to  make  it  flow;  i.e.,  the  machine  acts  as  a 
generator.  If  Eg  is  less  than  E  then  the  line  E.M.F.  forces  cur- 
rent to  flow  through  the  armature  of  No.  2  against  its  generated 
E.M.F.  and  No.  2  acts  as  a  shunt  motor,  drawing  power  from  the 
line  instead  of  furnishing  power  to  it. 

If  switches  N2  and  P2  are  closed  when  Eg  is  not  nearly  equal 
and  opposite  to  E,  then  an  excessive  current  will  necessarily  flow 
so  that  the  equation  may  be  satisfied.  This  rush  of  current  dis- 
turbs the  line  voltage,  may  blow  fuses  and  circuit  breakers  and 
even  injure  the  machine.  It  is  always  advisable,  therefore,  to 
have  Eg  as  nearly  equal  to  E  as  possible. 

It  has  been  mentioned  that  Eg  must  be  opposite  to  E-,  this 
is  sometimes  expressed  by  saying  that  the  machines  must  be  con- 


74  TESTING  OF  ELECTRICAL  MACHINERY 

nected  in  the  proper  polarity.  That  this  is  a  very  necessary  con- 
dition is  almost  self-evident. 

Suppose  that  Eg  were  not  opposite  to  E.  As  soon  as  switches 
P2  and  N2  are  closed,  both  E  and  Eg  would  tend  to  force  current 
in  the  same  direction,  through  the  circuit  composed  of  the  two 
armatures  and  the  bus-bars.  As  the  resistance  of  this  circuit  is 
extremely  low,  the  conditions  would  be  one  similar  to  a  dead  short 
circuit  on  each  machine.  To  test  for  polarity  N2  may  be  closed 
and  a  voltmeter  of  range  at  least  twice  the  voltage  of  the  machines, 
connected  across  switch  P2.  If  machine  No.  2  is  connected  with 
proper  polarity,  the  voltmeter  reading  will  be  E  —Eg;  if  improperly 
connected,  the  voltmeter  will  read  E-\-Eg. 

Another  way  of  testing  for  polarity  is  to  connect  a  voltmeter 
(one  of  range  equal  to  machine  voltage  only  is  necessary)  to  the 
points  aa'  and  then  transfer  the  voltmeter  leads,  'without  inter- 
changing them  to  the  points  W .  If  the  voltmeter  deflects  the  same 
way  in  both  cases  the  polarity  is  correct;  if  not,  the  connections 
of  machine  No.  2  to  the  bus-bar  switches  must  be  reversed  or  else 
the  generator  must  be  forced  to  build  up  in  the  opposite  direction. 

The  question  of  division  of  load  between  the  two  machines  is 
now  to  be  analyzed.  When  the  bus-bars  voltage  is  supposed  con- 
stant and  the  load  current  is  taken  as  constant,  it  is  a  very  simple 
matter  to  see  from  Eq.  (17)  that  the  current,  I2)  depends  directly 
upon  the  value  of  Eg.  If  Eg  is  increased  while  E  remains  constant 
I2  must  increase.  The  output  of  machine  No.  2  depends  directly 
upon  the  value  of  its  generated  voltage. 

If  machine  No.  2  has  been  brought  up  to  rated  speed,  its 
voltage  adjusted  to  be  equal  to  the  line  voltage  and  the  polarity  test 
satisfied,  N2  and  P2  may  be  closed  and  no  current  will  flow  through 
its  armature.  If  now  its  generated  voltage  is  increased,  by 
decreasing  the  resistance  of  the  shunt  field  circuit,  it  will  begin 
to  deliver  current  to  the  line,  the  amount  depending  upon  how 
much  Eg  is  increased.  The  division  of  load  between  two  shunt 
generators  is  thus  always  at  the  command  of  the  operator;  by 
proper  field  adjustment  the  load  may  be  shifted  from  one  machine 
to  the  other  at  will. 


PARALLEL  OPERATION  OF  SHUNT  GENERATORS 


75 


if  the  load  is  once  equally  divided,  the  question  arises  whether 
the  division  will  remain  equal  as  the  load  varies.  This  is  an  impor- 
tant point  to  investigate,  as  it  is  very  desirable  that  after  the  ma- 
chines are  once  adjusted  for  proper  load  division,  the  division 
should  not  change  as  the  load  fluctuates.  If  the  load  is  once 
properly  divided  (proportionate  to  the  relative  capacities  of  the 
machines)  the  division  will  only  be  automatically  maintained  if 
the  external  characteristics  of  the  two  machines  are  coincident 
throughout  the  range  of  operation. 


Current 

FIG.  35 

Suppose  the  load  to  be  zero  and  the  voltages  of  the  two  machines 
adjusted  to  be  equal.  The  supposed  external  characteristics 
of  the  machines  are  given  in  Fig.  35.  Now.  as  the  machines  are 
connected  to  a  common  bus,  their  terminal  voltages  must  always  be 
equal.  But  it  is  seen  that  if  the  two  machines  are  to  have  the  same 
voltage  with  a  certain  load  (/i+/2)  that  machine  No.  i  must  be 
furnishing  current  /i  and  machine  No.  2  current  /2.  So  that  from 
this  figure  it  is  seen  that  as  the  load  varies,  (field  rheostats  left 
in  fixed  position)  the  division  of  load  will  change,  the  amount 
of  change  depending  upon  how  far  the  external  characteristics 
separate. 


76  TESTING    OF   ELECTRICAL   MACHINERY 

If  the  bus-bar  voltage  is  allowed  to  vary  as  the  load  changes  the 
division  of  load  between  the  two  machines  will  change  inexactly  the 
same  manner  as  shown  in  the  previous  analysis,  but  the  effects  noted 
will  not  take  place  to  the  same  degree.  In  Eq.  (17)  £  will  generally 
decrease  as  load  is  increased  and  this  will  tend  to  make  machine 
No.  2  take  more  load  for  a  given  Eg  than  if  E  remained  constant. 

Connect  the  shunt  generators  to  be  operated  in  parallel  as  in 
Fig.  34.  Attach  one  of  the  machines  to  the  bus-bars  and  load  it 
with  lamps.  Then  bring  the  second  machine  up  to  speed,  build  up 
its  voltage  to  that  of  the  bars,  and  having  made  certain  that  the 
terminals  of  the  machine  will  be  properly  connected  as  regards 
polarity,  close  the  line  switches.  Strengthen  the  field  of  the  incom- 
ing machine  until  it  takes  half  of  the  load  and  then  weaken  the 
field  of  the  first  machine  until  it  is  supplying  no  current.  Then 
further  weaken  the  field  of  the  first  machine  until  it  operates  as  a 
motor.  This  fact  will  be  indicated  by  the  armature  ammeter 
reversing.  Note  that  the  shunt  field  current  continues  to  flow  in 
the  same  direction.  Then  bring  the  current  in  the  first  machine 
to  zero  and  disconnect  it  from  the  bars.  Continue  this  procedure 
until  sufficient  practice  has  been  had ,  in  putting  machines  on  and 
off  the  bars  and  in  throwing  the  load  back  and  forth. 

(a).  Bring  both  generators  in  parallel  upon  the  bars  at  rated 
voltage  with  no  armature  current  in  either  machine.  Then  add 
load  in  equal  steps  up  to  the  full  load  value  of  each  machine,  keep- 
ing it  equally  divided  between  the  two  machines,  voltage  constant 
at  rated  value  and  speeds  constant.  Read  all  meters,  recording 
readings  in  a  log  as  in  table  X. 

(b).  Starting  as  before  with  rated  voltage  and  no  load  upon  either 
machine,  add  load  to  the  system  but  allow  the  total  current  to 
divide  between  the  machines  as  it  will.  Permit  the  voltage  to 
vary,  but  keep  speeds  constant  at  rated  value. 

After  run  b,  with  about  half  load  on  each  machine,  investigate 
the  effect  of  shifting  the  brushes  of  one  machine  forward  and 
backward.  Be  careful  not  to  shift  too  far  if  the  machines  are 
equipped  with  commutating  poles. 

Curves.     Upon  one  sheet  of  cross-section  paper  plot  three 


PARALLEL  OPERATION   OF  SHUNT   GENERATORS 


77 


^3 

0 

N 

K<i"3 

X 

.2 

° 

"o 
H 

O 

5  v> 

\o 

O 

«s 

.«i 

o 

Ov 

^  cx 

X 

«<S 

. 

s 

0 

oo 

(X 

i 

'S  u 

o 

S  S 

o 

"rt  -*-> 

eg 

M 

o 

4)    IH 

«  3 

w 

wo 

j 

« 

s 

.4^ 

10 

^  a 

X 

M| 

0 

« 

\ 

1 

8 

^ 

§ 
0 

f*5 

If 

O 

eg 

M 

»<  3 

WO 

|s 

WW 

78  TESTING    OF   ELECTRICAL  MACHINERY 

curves  (one  for  run  a  and  two  for  run  b),  between  terminal  E.M.F.'s 
(ordinate)  and  individual  external  currents.  Also  plot  four 
curves  (two  for  each  run)  between  shunt  field  currents  (ordinates) 
and  individual  external  currents. 

Conclusions.  What  precautions  must  be  taken  in  connecting 
shunt  generators  for  parallel  operation?  Why,  if  left  without 
adjustment,  do  the  machines  not  divide  the  load  in  proportion 
to  their  rated  outputs?  Why  is  the  parallel  operation  of  shunt 
generators  rather  impractical  from  a  commercial  standpoint? 
Explain  the  form  of  the  curves  obtained.  Explain  the  effects  of 
shifting  the  brushes  of  one  of  two  shunt  generators  operating  in 
parallel. 


EXPERIMENT   XI 

Parallel  Operation  of  Compound  Generators.  The  parallel 
operation  of  compound  generators  is  in  many  respects  similar  to 
the  operation  of  shunt  generators;  there  are,  however,  a  few  added 
precautions  to  be  noted  before  they  may  be  connected  to  the  same 
bus-bars. 

In  Fig.  36  are  shown  the  connections  of  two  compound  gen- 
erators intended  for  parallel  operation.  Let  us  again  consider 


® 


FIG.  36 

No.  i  machine  as  connected  to  a  load  and  that  whatever  manipula- 
tion is  carried  out  with  generator  No.  2,  the  bus-bar  voltage  will 
again  be  considered  constant.  Let  us  also  for  the  time  being, 
suppose  that  the  connection  e\  e2  is  not  present.  It  is  now  desired 
to  connect  machine  No.  2  to  the  same  bus-bars,  in  order  to  have  it 
share  the  load.  The  same  precautions  as  to  polarity  and  voltage 
must  also  apply  in  this  case  as  in  the  operation  of  shunt  generators 
except  that  if  the  second  generator  comes  in  with  its  polarity 
opposite  to  that  of  the  bus-bars,  its  residual  magnetism  must  be 
reversed  so  that  it  will  build  up  with  the  proper  polarity.  Revers- 
ing the  machine  connections  at  the  bus-bars  will  result  in  a 
short  circuit  when  the  equalizer  switches  are  closed  and  reversing 
the  armature  connections  alone  will  make  the  machine  act  as 

79 


80 


TESTING  OF  ELECTRICAL  MACHINERY 


a  differential  generator.  Accordingly  let  us  bring  up  the  voltage 
of  generator  No.  2  to  the  same  value  as  that  of  the  bus-bars  and 
close  the  switches  N2  and  P2.  The  generator  may  then  be 
made  to  assume  load  as  before  by  strengthening  its  field. 

The  machines  are,  however,  now  operating  under  a  condition 
of  unstable  equilibrium.  Let  us  suppose  that  the  speed  of 
No.  i  machine  were  momentarily  raised  and  its  generated  voltage 
thereby  raised  for  an  instant.  This  would  immediately  mean  that 
it  would  take  a  slightly  greater  load,  depriving  No.  2  of  some. 
The  additional  current  flowing  through  the  series  field  of  the  first 
machine  would  cause  its  generated  voltage  to  rise  still  higher 
while  that  of  No.  2  would  be  decreased.  This  action  is  likely 
to  continue  until  No.  i  is  supplying  all  of  the  load  and  No.  2 


Bus  Bar 


—  Bus  Bar 


-\-BusBar 
FIG.  37 


-i-Bus  Bar 
FIG.  38 


none  of  it,  and  it  is  also  likely  that  the  generated  voltage  of  No.  2 
would  drop  below  the  voltage  of  the  buses  so  that  the  latter  would 
take  current  from  the  bus-bars  and  operate  as  a  motor.  This 
follows  as  before  if  we  consider  the  equation  E=E0—IR. 

We  have  seen  that  in  the  case  of  shunt  generators  in  parallel, 
no  danger  would  result  if  one  machine  operated  as  a  motor.  In 
the  case  of  compound  generators  this  action,  with  the  conditions 
as  assumed,  is  really  dangerous.  In  Fig.  37  are  shown  two  com- 
pound generators  operating  in  parallel,  both  acting  as  generators. 
The  current  through  the  armature  and  series  field  flows  from  the 
negative  to  the  positive  bus  as  indicated  by  the  long  arrows,  while 
the  shunt  field  currents  flow  from  the  positive  to  the  negative 
terminals  of  the  machine  as  indicated  by  the  short  arrows.  As 


PARALLEL  OPERATION  OF  COMPOUND  GENERATORS          81 

the  machines  are  compound  wound  generators,  the  M.M.F.'s  of 
the  shunt  and  series  fields  are  added  and  the  arrows  point 
in  the  same  direction  to  indicate  this  fact.  When  No.  2  machine 
operates  as  a  motor  (Fig.  ^8),  the  current  in  its  armature  and 
series  field  reverses  flowing  from  the  positive  to  the  negative  bus. 
The  current  through  its  shunt  field,  however,  does  not  reverse 
but  flows  as  before,  so  that  the  M.M..F.'s  of  the  two  fields  are 
opposed,  which  results  in  the  net  flux  of  the  machine  being 
reduced. 

This  in  turn  results  in  two  things.  First,  a  machine  whose 
field  is  weakened  tends  to  increase  its  speed;  this  is  somewhat 
difficult  in  this  case,  since  the  machine  then  tends  to  drive  its  prime 
mover,  and  it  is  generally  unable  to  accelerate  it  very  much.  The 
second  result  of  the  weakened  field,  is  an  actual  decrease  in  gene- 
rated E.M.F.  or  what  it  really  is,  since  the  machine  is  operating  as 
a  motor,  a  decreasedC. E.M.F.  This  is  due  to  the  fact  that  the 
machine  cannot  raise  its  speed  fast  enough,  and,  besides,  the  action 
is  cumulative.  For  if  the  C. E.M.F.  is  slightly  decreased,  more 
current  flows  through  the  series  field  and  armature,  which  makes  the 
series  field  stronger  and  the  net  flux  weaker.  Thus  the  more 
current  the  machine  takes  the  more  its  field  is  weakened  and  so 
on.  This  goes  on  so  rapidly  that  the  speed  has  no  chance  to  catch 
up  and  keep  the  C. E.M.F.  up  to  the  required  value,  the  net 
result  being  that  No.  2  forms  a  short  circuit  for  No.  i,  blowing 
fuses  and  circuit  breakers  and  disturbing  the  system.  Often  the 
inrush  current  into  No.  2  will  be  so  great  as  to  cause  the  series 
field  to  become  so  strong  that  it  overpowers  the  shunt  field  and 
actually  reverses  the  polarity  of  the  residual  magnetism  of  the 
machine. 

To  prevent  this  action  and  make  the  parallel  operatic:  of 
compound  generators  stable,  a  connection  e\e^  termed  the 
"  equalizing  "  bus-bar,  is  employed,  and  it  will  be  noticed  that  it 
joins  the  machines  between  their  armatures  and  series  fields. 
This  connection  prevents  the  reversal  of  the  current  through  the 
series  field  of  the  machine  operating  as  a  motor,  which  is  readily 
seen  if  we  suppose  switches  EI  and  E2  closed  and  machine  No.  2 
operating  as  a  motor.  Its  armature  current  then  flows  from  the 


82  TESTING  OF  ELECTRICAL  MACHINERY 

positive  bus,  but  when  it  arrives  at  the  point  x  it  has  a  choice  of 
either  passing  on  through  the  series  field  of  No.  2  along  the- negative 
bus  and  through  the  series  field  of  No.  i,  or  flowing  directly 
along  the  equalizer  connection.  As  this  latter  is  always  of  very 
low  resistance,  the  current  will  take  this  path.  Machine  No.  2 
is  thus  in  reality  operating  as  a  shunt  motor,  which  we  have  seen 
is  not  accompanied  by  any  dangerous  conditions. 

As  the  equalizer  is  of  very  low  resistance,  the  IR  drop  of  the 
two  series  fields  must  under  all  conditions  be  equal.  If  the  load 
of  No.  i  increases,  the  IR  drop  of  series  field  No.  i  tends  to  increase, 
but  this  can  only  increase  if  a  corresponding  increase  in  the  drop 
of  series  field  No.  2  takes  place  and  this  means  more  current 
through  the  series  field  of  No.  2.  If,  therefore,  No.  i  tends  to 
increase  its  load  and  so  raise  its  voltage,  No.  2  will  also  raise  its 
voltage  due  to  the  action  of  the  increased  current  through  its 
series  field,  which  flows  through  the  equalizer  connection  from  No. 
i.  The  division  of  the  load  between  the  two  machines  thus  tends 
to  remain  more  or  less  constant.  It  follows  from  the  above  that  the 
resistances  of  the  two  series  fields  must  be  inversely  proporional 
to  the  full  load  current  outputs  of  the  two  machines,  for  if  this  is 
not  so,  the  series  field  currents  would  not  be  of  their  proper  values 
when  the  armature  currents  are  correct.  It  is  also  necessary 
that  the  two  machines  have  the  same  characteristics,  that  is,  each 
must  have  the  same  degree  of  compounding  when  running 
separately.  If  they  differ  in  this  respect,  the  machines  will  not 
share  the  load  equally. 

The  sequence  of  closing  switches  when  putting  machines 
upon  the  bus-bars  is  also  very  important.  Again  consider  machine 
No.  i  supplying  current  to  a  load.  When  it  becomes  necessary 
to  parallel  No.  2,  we  might  first  bring  it  up  to  the  bus  voltage  by 
shunt  field  regulation  and  then  close  E2  and  N2.  Current  would 
then  flow  through  the  series  field  of  No.  2,  and  raise  its  generated 
voltage  somewhat  above  that  of  the  bus-bars.  This  would  require 
further  shunt  field  regulation,  so  that  it  is  better  to  first  close  switches 
N2  and  E2  and  then  when  the  machine  has  been  brought  up  to 
the  bus-bar  voltage  by  shunt  field  regulation,  P2  can  be  closed. 


PARALLEL   OPERATION    OF   COMPOUND  GENERATORS       S3 


ti 

o 


wo 


II 

X  3 
WO 


WW 


84  TESTING  OF  ELECTRICAL  MACHINERY 

Switch  P%  must  never  be  closed  unless  the  machine  is  up  to  voltage 
and  must  always  be  the  last  one  thrown  in. 

Load  is  then  put  upon  the  machine  by  strengthening  its  shunt 
field.  To  remove  No.  2  machine  from  the  bus-bars,  first  reduce 
its  armature  current  to  zero  by  means  of  shunt  field  regulation 
and  then  open  switch  P%  and  then  Ni  and  E2.  Always  open  P2 
first.  The  prime  mover  of  No.  2  can  then  be  stopped  and  the 
machine  shut  down. 

Operate  the  machines  with  the  equalizer  in,  as  was  done  in 
Ex.  10,  bringing  a  machine  upon  the  bus-bars,  transferring  the 
load  from  one  machine  to  another  and  removing  one  machine. 
Also  cause  one  machine  to  operate  as  a  motor  and  note  results. 

Make  two  tests,  taking  about  twelve  readings  in  each  run, 
including  a  no-load  setting,  and  recording  readings  in  a  log  as  in 
Table  XI. 

(a)  Start  with  the  machines  in  parallel,  but  with  no  load  upon 
either  and  gradually  increase  the  load  up  to  rated  value,  keeping 
system  E.M.F.,  and  speed  constant  at  rated  value  and  current 
equally  divided  between  the  two  machines. 

(b)  Start  as  before  and  add  load  and  keep  only  speeds  constant. 
Allow  terminal  E.M.F.  to  vary  and  the  load  to  divide  as  it  will. 

After  run  (b)  with  half  load  on  each  machine,  note  the  effect 
of  shifting  brushes.  Do  not  shift  too  far,  if  the  machines  are 
equipped  with  commutating  poles. 

Curves.  Plot  three  curves  (one  for  run  a  and  two  for  run  b)  be- 
tween system  E.M.F.  (ordinates)  and  individual  external  currents. 
Upon  the  same  sheet  of  cross-section  plot  four  curves  (two  for  each 
run)  between  shunt  field  currents  and  individual  external  current. 

Conclusions. — Explain  what  is  likely  to  happen  if  compound 
generators  are  operated  without  an  equalizer  connection.  What 
are  the  functions  of  the  equalizer  connection  and  how  does  it  carry 
them  out?  What  is  the  proper  sequence  of  closing  switches  and 
why  is  this  sequence  absolutely  necessary?  Is  the  parallel  opera- 
tion of  compound  generators  entirely  satisfactory?  If,  with  the 
generators  connected  as  in  Fig.  36,  test  shows  that  the  polarity 
of  the  incoming  machine  is  opposite  to  that  of  the  bus-bars, 
what  must  be  done? 


EXPERIMENT   XII 

Commutating  Pole  Motor  and  Generator.  In  motors  and 
generators  not  equipped  with  commutating  poles,  commutation 
is  improved  by  brush  shifting.  As  a  coil  on  the  armature. is 
commutated,  its  current  must  reverse,  or  die  down  to  zero 
and  build  up  in  the  reverse  direction.  In  order  that  there 
shall  be  no  sparking  the  rate  of  change  of  the  current  must  be 
such  that  the  current  has  built  up  to  its  final  value  when  the 
brush  and  commutator  bar  separate.  However,  due  to  the  fact 
that  the  coil  being  commutated  possesses  some  self-induction, 
the  changing  current  generates  a  counter  E.M.F.  which  opposes 
the  change.  As  a  result  the  current  may  not  have  built  up  to  its 
final  value  when  brush  and  bar  part  company  and  a  spark  is 
formed. 

If  during  commutation,  while  the  changing  current  generates 
a  counter  E.M.F.  of  self-induction,  the  coil  can  be  made  to  cut 
flux  and  generate  a  voltage  equal  and  opposite  to  the  C. E.M.F. 
of  self-induction,  commutation  will  be  satisfactory.  This,  as 
was  noted,  is  accomplished  by  moving  the  brushes.  Since  the 
direction  of  the  E.M.F.  required  to  overcome  the  C. E.M.F. 
of  self-induction  must  be  in  the  same  direction  as  that  of  the  final 
current,  it  follows  that  the  brushes  must  be  shifted  forward  in 
the  generator  and  backward  in  the  motor. 

To  express  the  conditions  just  stated  we  may  write 

dl  _     d<t>c 

LTt~  fV^TJ  '    '    ' 

where  L  =  coefficient  of  self-induction  of  the  coil    being  commu- 
tated; 

7  =  armature  current  per  coil ; 
<j>c  =  flux  cut  by  short-circuited  coil  under  pole  tip ; 
/  =  time  of  commutation; 
N  =  number  of  turns  in  the  coil. 

85 


86  TESTING   OF   ELECTRICAL   MACHINERY 

Evidently  as  the  load  on  the  machine  varies,  the  current  / 
will  change  and  therefore  the  value  of  the  left-hand  term  of  the 
expression.  In  order  to  maintain  equality  and  hence  good  com- 
mutation, the  value  of  flux  <£c,  which  the  coil  cuts  must  be 
changed,  and  as  this  depends  upon  the  position  of  the  brushes, 
there  is  a  different  position  of  the  brushes  for  each  load.  In 
small  motors  this  is  not  attempted,  the  brushes  being  set  to  that 
position  which  gives  the  best  average  commutation.  The  limi- 
tation of  the  method  on  overloads  is  the  fact  that  armature 
reaction  causes  distortion  of  the  flux  and  weakening  the  very 
pole  tip  toward  which  the  brushes  are  shifted  and  under  which 
the  commutating  flux  is  sought.  This  limitation  was  particularly 
noted  in  Exp.  V.,  when  the  method  of  raising  the  speed  of  a 
shunt  motor  by  field  weakening  was  investigated.  As  the  field 
flux  is  reduced,  armature  reaction,  which  is  the  effect  of  armature 
M.M.F.  on  field  M.M.F.,  is  able  to  cause  greater  distortion  than 
when  the  field  is  normal. 

By  the  use  of  commutating  poles  the  same  result  is  obtained. 
While  the  method  of  shifting  brushes  moves  the  short-circuited 
coil  to  the  necessary  commutating  flux,  the  commutating  pole 
brings  the  flux  to  the  coil.  From  Eq.  (18)  it  is  evident  that,  to 
perform  its  function,  the  flux  from  the  commutating  pole  must 
always  be  proportional  to  the  armature  current,  and  for  this 
reason  the  commutating  pole  is  excited  by  putting  its  winding 
in  series  with  the  armature.  It  is  also  so  designed  that  up  to 
fair  overload  it  will  not  become  saturated. 

It  should  be  noted  that  whereas  the  commutating  pole 
neutralizes  the  M.M.F.  of  the  armature  in  the  immediate  region 
where  its  flux  enters  and  leaves  the  armature,  complete  neu- 
tralization of  the  armature  M.M.F.  is  not  possible  by  the  use 
of  commutating  poles,  inasmuch  as  the  space  distribution  of  the 
armature  and  commutating  pole  M.M.Fs.  are  entirely  different. 

In  Fig.  39  is  shown  the  flux  distribution  in  a  commutating 
pole  machine  for  a  given  load.  As  the  amount  of  flux  from  the 
commutating  poles  is  proportional  to  the  load,  the  flux  distribu- 
tion throughout  the  machine  will  be  different  for  each  load.  It 


COMMUTATING   POLE   MOTOR   AND    GENERATOR 


87 


may  be  seen  that  the  commutating  poles  do  not  prevent  the  flux 
from  crowding  into  the  main  pole  tips.  This  does  practically 
no  harm;  the  principal  thing  to  obtain  is  the  proper  field  for 
commutation  in  the  turns  short- 
circuited,  which,  for  normal  opera- 
tion, should  be  directly  under  the 
commutating  poles. 

In  Fig.  40  the  armature  of  a  com- 
mutating pole  generator  is  shown 
with  the  brushes  in  the  proper  posi- 
tion. Under  the  main  and  commu- 
tating poles  voltages  are  generated 
in  the  various  conductors,  being 
indicated  by  dots  and  crosses  inside  the  conductors.  The 
currents  the  conductors  are  carrying  under  load  are  indicated 
by  dots  and  crosses  outside.  A  dot  indicates  a  voltage  or 


C.W.  Rotation 

for 
Generator 


,  c.c.w. 

Rotation 
fbrMotor 


FIG.  39 


a' 


FIG.  41 


current  toward  the  observer.  It  is  customary  to  make  the 
width  of  the  commutating  poles  slightly  greater  than  the  dis- 
tance moved  over  by  a  slot  while  the  coils  in  it  are  undergoing 
commutation.  Even  if  voltages  are  induced  under  the  com- 
mutating poles  by  conductors  before  and  after  they  are  short- 
circuited  by  the  brushes,  their  effects  neutralize.  Whatever 
voltage  is  induced  under  a  north  commutating  pole  on  one 


88  TESTING   OF   ELECTRICAL   MACHINERY 

side  of  the  line  of  brushes  a,a',  is  equal  and  opposite  to  that 
induced  under  the  south  commutating  pole. 

If  now  the  brushes  are  shifted  backward  in  the  generator, 
as  indicated  in  Fig.  41,  the  effect  is  to  cause  a  certain  amount  of 
compounding.  One  way  of  regarding  it  is  that  on  the  same  side 
of  the  brush  line  there  is  a  series  field  pole  of  the  same  polarity 
as  the  main  pole.  As  load  increases,  the  commutating  pole  is 
strengthened  and  the  voltage  tends  to  rise.  A  better  way  of 
considering  the  action  is  that  the  voltages  induced  under  the 
commutating  poles,  are  now  no  longer  used  to  oppose  the 
C.E.M.Fs.  of  self-induction  in  the  short-circuited  coil  since 
shifting  the  brushes  has  moved  this  coil  out  from  under  the 
commutating  pole.  The  voltages  are  then  added  to  those 
induced  under  the  main  poles,  causing  more  or  less  compounding 
depending  on  how  far  the  short-circuited  turn  has  been  moved. 
Only  a  very  slight  forward  shift  of  the  brushes  will  change  the 
amount  of  compounding  of  a  generator  to  a  considerable  degree. 
The  function  of  the  commutating  poles  being  to  improve  com- 
mutation brush  shifting  is  never  relied  upon  to  effect  com- 
pounding, for  it  is  evident  that  if  the  brushes  are  shifted  from  the 
neutral  point,  commutation  will  suffer. 

For  a  forward  shift  of  the  brushes  the  reverse  takes  place, 
for  the  voltages  induced  under  the  commutating  poles  will  now 
be  opposite  to  those  induced  under  the  main  poles  on  the  same 
side  of  the  brush  line  and  the  voltage  of  the  generator  will  fall 
off  rapidly  with  load. 

In  commutating  pole  motors,  shifting  brushes  affects  the 
speed.  In  Figs.  42  and  43  are  shown  the  armature  of  a  shunt 
motor  in  which  the  direction  of  the  C.E.M.Fs.  is  indicated  by 
dots  and  crosses  inside  the  conductors  and  the  direction  of  the 
currents  by  dots  and  crosses  outside.  Whatever  voltages  are 
generated  under  the  commutating  poles  when  the  brushes  are 
shifted,  will  be  added  or  subtracted  from  the  C.E.M.Fs.  gen- 
erated under  the  main  poles.  In  Fig.  42  the  brushes  are  shifted 
forward,  so  that  as  load  is  added,  more  conductors  generating 
C.E.M.F.,  the  speed  will  fall  more  than  that  determined  by  the 


COMMUTATING   POLE   MOTOR  AND   GENERATOR 


89 


required  IR  drop.  In  other  words,  the  machine  has  the  speed 
characteristic  of  a  compound  motor.  We  may  also  regard  the 
action  as  in  the  generator,  as  the  addition  of  a  series  field  on  the 
same  side  of  the  brush  line,  the  flux  increasing  with  load.  Or  we 
may  consider  that  the  effective  number  of  armature  conductors 
has  been  increased  (page  57). 


N 


FIG.  42 


Dots  and  Crosses 
inside  conductors 
represent  C.  E.  M.  Fs. 


FIG.  43 


With  a  backward  shift  of  the  brushes  (Fig.  43)  a  shunt  motor 
will  have  the  speed  characteristic  of  a  differential  motor,  the 
speed  remaining  nearly  constant  or  even  increasing  with  load. 
With  any  but  a  slight  shift  backward,  particularly  with  weak 
fields,  the  motor  is  in  a  state  of  unstable  equilibrium.  If  a 
sudden  surge  of  armature  current  occurs,  the  effects  of  the 
commutating  poles  will  cause  considerable  reduction  of  the 
C.E.M.F.  which  will  cause  further  increase  in  the  armature 
current.  Meanwhile  the  speed  will  rapidly  increase  and  the 
machine  is  apt  to  run  away.  The  behavior  under  these  condi- 
tions is  analogous  to  the  operation  of  compound  generators 
in  parallel  without  any  equalizer  connection. 

//  is  therefore  inadvisable  to  shift  the  brushes  of  commutating 
pole  motors.  Commutation  will  be  impaired  and  if  differential 
or  compound  speed  characteristics  are  desired,  it  is  better  to 
obtain  them  by  using  series  fields.  Furthermore  in  the  case  of 
motors  which  are  operated  in  either  direction  of  rotation,  if  the 


90  TESTING    OF   ELECTRICAL   MACHINERY 

brushes  are  not  on  the  neutral  point,  the  motors  will  have  a  dif- 
ferent speed  in  one  direction  than  in  the  other  for  the  same  load. 
In  fact  this  is  one  method  of  determining  the  proper  position 
for  the  brushes. 

The  use  of  commutating  poles  also  makes  possible  the  varia- 
tion of  the  speed  of  a  shunt  motor  through  wide  limits  by  field 
weakening.  In  the  shunt  motor  without  commutating  poles,  it 
was  pointed  out  (page  36)  that  the  practical  limit  to  weakening 
the  field  is  imposed  by  sparking  at  the  brushes,  due  to  the  fact 
that  with  the  weakened  field,  armature  reaction  is  able  to  distort 
the  field  to  such  an  extent  that  there  is  no  commutating  flux. 
We  saw  in  Eq.  (18)  that  so  long  as  the  commutating  flux  (<£c), 
supplied  now  by  the  commutating  poles,  is  proportional  to  the 
armature  current,  commutation  will  remain  satisfactory.  It 
was  also  stated  that  even  though  the  main  flux  is  distorted  by 
the  use  of  commutating  poles,  no  great  practical  harm  results. 
So  as  the  speed  is  varied  by  field  weakening,  commutation  will 
still  remain  good,  for  while  the  C.E.M.F.  of  self-induction 
increases  as  the  time  of  commutation  is  reduced,  the  rate  of 
cutting  of  the  commutating  flux  is  also  increased  by  the  higher 
speed. 

There  results  the  "adjus table  speed"  shunt  motor,  in  which 
the  speed  is  adjusted  by  shunt  field  current  variation,  the 
motor  once  adjusted  having  the  speed  characteristic  of  an 
ordinary  shunt  machine  with  variation  of  load.  Adjustable 
speed  motors  are  built  for  ranges  of  speed  of  from  2  to  i  to  as 
high  as  6  to  i.  In  a  4  to  i  speed  motor  the  highest  speed  per- 
missible is  four  times  the  lowest  value.  Such  motors  are  usually 
provided  with  special  starting  rheostats  which  are  a  combination 
of  an  ordinary  starting  rheostat  and  a  field  rheostat. 

A  type  of  starting  rheostat  for  an  adjustable  speed  motor  is 
shown  in  Fig.  44.  A  peg  B,  inserted  into  a  flat  metal  disc  D, 
is  forced  by  the  tension  of  a  spiral  spring  at  the  pivot  P,  up 
against  the  main  arm  C,  which  in  turn  is  forced  up  against  the 
spring  stop  5.  When  arm  C  is  grasped  by  the  handle  H  and 
moved  upward  to  the  right,  it  pushes  against  peg  B,  forcing  the 


COMMUTATING   POLE   MOTOR   AND    GENERATOR 


91 


disc  D  around.  When  arm  C  approaches  the  end  of  its  travel, 
peg  B  comes  up  against  a  latch  L  rigidly  attached  to  arm  E 
which  is  pivoted  at  V.  As  B  continues  to  move,  it  pushes  arm  E 
around  so  that  it  rotates  in  a  counter-clockwise  direction  until 
the  keeper  T  comes  up  against  the  no- voltage  release  R,  which 
when  excited  holds  the  keeper  T.  While  arm  E  rotated,  the 


FIG.  44 

finger  of  latch  L  swung  around  so  as  to  grasp  peg  B,  so  that  the 
no-voltage  release  also  holds  the  disc  D  in  position.  Tension  is 
now  removed  from  arm  C  and  it  can  be  moved  back  and  forth 
over  the  outer  series  of  resistance  buttons  at  will,  remaining 
wherever  left.  Whenever  the  no-voltage  release  lets  go,  spring 
tension  at  pivot  V  and  also  at  P,  force  arm  E  and  the  disc  to 
rotate  clockwise  and  counter-clockwise,  respectively.  In  the 
course  of  its  travel  peg  B  strikes  arm  C,  forcing  it  up  against 


92  TESTING   OF   ELECTRICAL   MACHINERY 

stop  S.  At  the  same  time  a  second  peg  W  comes  up 
against  the  under  side  of  latch  Z,,  forcing  arm  E  against  its 
stop  S'. 

When  the  main  switch  is  closed  the  shunt  field  circuit  is  made, 
current  flowing  in  at  terminal  M,  to  pivot  P,  pivot  F,  over  arm 
E  to  button  G,  button  Q,  terminal  F,  through  the  field  and  back 
to  the  line.  If  arm  C  is  now  moved  up  to  make  contact  with 
button  U,  current  flows  from  pivot  P  to  button  V  through  the 
starting  resistance  to  button  K,  to  button  /  to  terminal  A' 
and  through  the  armature.  The  motor  having  full  field  strength 
starts  to  rotate.  As  the  arm  continues  upward,  more  and  more 
•of  the  starting  resistance  is  cut  out.  When  arm  C  pushes  arm  E 
around,  it  will  be  seen  that  E  is  now  making  contact  with  button  / 
instead  of  G.  The  armature  current  will  now  flow  from  P  to  F, 
over  arm  E  to  J,  the  starting  resistance  being  thereby  short- 
circuited.  Field  current  now  flows  over  arm  C  to  button  Q 
and  as  arm  C  is  moved  backward  over  the  outer  series  of  buttons, 
more  resistance  is  inserted  into  the  field. 

If  the  experiment  is  to  be  performed  on  a  compound  gene- 
rator, determine  its  compound  characteristic  with  its  brushes 
in  the  proper  position  and  also  for  a  forward  and  a  backward 
shift  of  the  brushes.  Also  determine  the  external  characteristic 
as  a  shunt  generator  with  the  brushes  in  the  proper  position 
and  also  with  them  shifted  backwards.  It  is  advisable  that  the 
amount  of  brush  shift  be  determined  by  an  instructor. 

When  performing  the  experiment  on  an  adjustable  speed 
motor,  operate  it  with  the  commutating  field  properly  connected 
and  also  with  it  reversed.  Do  this  at  a  fairly  high  speed  with 
about  full  load.  With  the  commutating  field  correctly  con- 
nected and  the  brushes  properly  set,  make  a  brake  test  at  the 
lowest  speed  for  which  the  machine  is  intended,  taking  all  data 
necessary  to  obtain  its  speed  load  characteristic  and  efficiency. 
Repeat  in  a  second  run  starting  at  an  intermediate  speed  and 
in  a  third  run  at  the  highest  speed  permissible.  Have  the 
brushes  shifted  forward  by  an  instructor  and  make  a  fourth  run 
starting  at  the  same  intermediate  speed  used  in  run  2.  Have 


COMMUTATING   POLE   MOTOR   AND   GENERATOR  93 

the  brushes  shifted  backward  by  an  instructor  and  make  a  fifth 
run  starting  again  as  in  run  2. 

Caution.  Because  of  the  possible  danger  that  the  machine 
may  run  away,  it  is  advisable  that  each  member  of  the  squad 
knows  how  to  quickly  and  conveniently  shut  the  machine  down 
if  it  starts  to  run  away. 

Curves.  Plot  on  one  sheet,  curves  of  efficiency,  speed  and 
torque  against  horse-power  output  from  the  results  of  runs 
i,  2  and  3.  On  a  second  sheet  plot  curves  between  speed  and 
H.P.  output  from  the  results  of  runs  3,  4  and  5. 

Conclusions.  Explain  how  commutating  poles  improve 
commutation.  What  happens  if  the  commutating  pole  winding 
is  incorrectly  connected?  What  is  the  result  of  shifting  brushes 
in  commutating  pole  machines?  Why  is  it  inadvisable  to  move 
the  brushes  of  such  machines  from  the  proper  neutral  position? 
Can  machines  be  designed  to  operate  successfully  with  only  half 
as  many  commutating  poles  as  main  poles?  If  in  the  parallel 
operation  of  generators  equipped  with  commutating  poles,  one 
machine  operates  as  a  motor,  is  its  commutating  field  properly 
connected  for  motor  operation?  Why?  For  what  types  of 
service  are  adjustable  speed  motors  adapted? 


EXPERIMENT  XIII 

Location  of  Faults  in  a    Direct    Current  Motor  or  Generator. 

The  location  of  the  following  faults  in  direct  current  motors  and 
generators  are  to  be  investigated : 

a.  Open  turn  in  armature. 

b.  Short-circuited  turn  in  armature. 

c.  Ground  in  field  windings. 

d.  Ground  in  armature  winding. 

a.  Open  Circuit  in  Armature.  Commercial  armature  wind- 
ings so  far  as  commutator  connections  are  concerned,  are  of  two 
types.  In  large  machines,  separate  commutator  risers  are  em- 


FIG.  45 


FIG.  46 


ployed  which  are  tapped  into  the  winding  at  the  outer  periphery 
of  the  armature  as  in  Fig.  45.  In  smaller  machines  the  ends 
of  the  coils  themselves  are  carried  down  into  the  commutator 
bars  as  in  Fig.  46.  In  the  first  type  of  winding  there  are  three 
ways  in  which  the  windings  may  open  up.  The  commutator 
riser  only  may  break  as  in  A,  a  break  may  occur  which  both 
opens  the  winding  proper  and  completely  disconnects  it  from 
the  riser  as  in  B,  or  only  the  winding  may  open  up,  say  at  the 
back  of  the  armature,  as  in  C. 

Break  A  will  show  no  decided  symptoms,  if  the  brushes  are 
wide  enough  to  cover  more  than  one  commutator  bar  and  the 
end  of  the  broken  commutator  riser  attached  to  the  winding 

94 


LOCATION  OF  FAULTS     IN  A  DIRECT-CURRENT  MOTOR       95 

does  not  make  contact  with  the  risers  on  either  side.  If  it  does, 
a  short-circuited  turn  results  which  will  be  treated  later.  If  the 
brush  does  not  cover  more  than  one  commutator  bar,  sparking 
will  result,  for  it  is  evident  that  when  the  brush  is  entirely  on 
bar  2  no  current  can  pass  to  the  winding.  A  moment  before  while 
the  brush  was  still  touching  bar  3  (supposing  the  armature  as 
moving  from  left  to  right),  current  was  flowing,  and  this  current 
is  broken  when  the  brush  leaves  bar  3.  A  pronounced  arc  occurs, 
due  to  the  self-induction  of  the  armature  winding,  which  causes 
blackening  and  pitting  of  bar  3.  In  both  cases  B  and  C  sparking 
will  result.  When  in  a  bi-polar  machine  the  break  in  the  winding 
is  between  brushes,  current  flows  around  the  other  armature 
circuit,  and  when  the  break  passes  under  a  brush  and  is  thus 
transferred  to  the  other  side  of  the  armature  a  spark  results, 
due  to  the  breaking  of  the  current  and  the  self-induction  of  the 
winding.  The  spark  will  only  occur  when  brush  and  bar  6 
(case  B)  separate,  causing  this  bar  to  blacken  and  pit. 

In  generators  sparking  and  failure  to  generate  are  the  im- 
portant symptoms.  In  motors  besides  sparking  there  will  also 
be  found  absence  of  proper  starting  torque  under  load  or  a 
tendency  to  turn  over  in  a  jerky  manner. 

A  very  simple  method  of  locating  an  open  armature  coil  is 
to  connect  several  dry  cells  across  the  brushes.  A  voltmeter 
with  a  range  somewhat  greater  than  the  open-circuit  voltage  of 
the  cells  is  then  successively  touched  to  adjoining  commutator 
bars.  When  touched  to  bars  on  the  side  of  the  armature,  where 
the  winding  is  not  open,  the  voltmeter  reading  will  be  quite 
small,  but  on  the  side  where  the  winding  is  open,  the  entire  volt- 
age of  the  cells  will  be  found  across  the  bars  between  which  the 
break  occurs. 

The  remedy  for  an  open  coil  is  to  find  it  and  close  it  by  splicing 
in  a  piece  of  suitable  wire.  It  is  better,  if  time  will  allow,  to  put 
in  an  entirely  new  coil.  Temporary  repairs,  however,  can  be 
made  at  the  commutator  by  electrically  connecting  commutator 
bars  4,  5,  and  6  (case  B,  Fig.  45),  by  soldering  or  screwing  on  a 
copper  strap.  Case  C  is  repaired  by  joining  bars  7  and  8. 


96  TESTING   OF   ELECTRICAL   MACHINERY 

The  latter  method  of  making  a  machine  operative  should  only 
be  used  where  it  is  absolutely  necessary  to  keep  it  in  operation, 
Proper  repairs  should  be  made  at  the  first  opportunity.  Care 
must,  however,  be  taken,  not  to  short-circuit  any  of  the  other 
coils. 

b.  Short-circuited  Coil.  In  both  motors  and  generators  a 
short-circuited  armature  coil  will,  as  it  rotates,  cut  the  field 
flux  and  therefore  generate  voltage,  which  will  cause  a  current 
to  circulate  in  the  coil.  This  current  will  prove  excessive  and 
if  left  flowing  for  any  length  of  time  will  heat  up  and  finally 
burn  out  the  coil,  the  heated  insulation  giving  out  a  bad  odor. 
Due  to  the  power  expended  in  heating  the  coil  a  motor  will  draw 
a  larger  current  and  a  generator  require  more  driving  power 
than  usual. 

A  short-circuited  coil  is  often  easily  located  by  feeling  the 
armature  all  over,  particularly  at  the  back,  after  operating  one 
or  two  minutes  and  noting  the  hottest  coil.  It  may  also  be 
located  at  the  commutator  by  the  method  given  above  for  de- 
tecting an  open  coil,  using  dry  cells  and  a  voltmeter.  It  is  evi- 
dent that  the  resistance  of  a  short-circuited  coil  is  less  than  the 
resistance  of  a  normal  coil,  so  that  the  reading  of  the  voltmeter 
across  the  commutator  bars  to  which  the  short-circuited  coil  is 
attached  will  be  very  small. 

To  repair  a  short-circuit  in  an  armature,  first  see  whether 
the  trouble  is  due  to  solder  or  copper  dust  between  commutator 
bars  or  risers,  in  which  case  remove  it.  If  the  difficulty  is  in 
the  coil  itself,  the  best  practice  is  to  put  in  a  new  one.  Tem- 
porary repairs  may  be  made  by  opening  the  coil  within  the  short- 
circuited  portion  and  then  bridging  the  commutator  as  for  an 
open  coil. 

c  and  d.  Grounds.  A  ground  in  a  machine  is  defined  as  an 
electrical  connection  between  the  windings  and  the  iron  portion 
of  the  machine,  that  is,  the  frame,  armature  laminations,  etc. 
They  are  due  to  worn  or  damaged  insulation,  so  that  the  copper 
wires  or  conductors  come  in  contact  with  the  iron  of  the  machine. 
It  is  evident  that  a  ground  may  also  be  of  high  or  low  resistance, 


LOCATION  OF  FAULTS  IN  A  DIRECT-CURRENT  MOTOR     97 

depending  on  how  good  the  contact  is  between  the  windings  and 
the  iron. 

If  a  machine  is  grounded  it  is  essential  that  the  ground  be 
removed  by  finding  it  and  repairing  the  worn  or  damaged  insu- 
lation. While  the  machine  is  grounded,  danger  to  the  attend- 
ants exists,  due  to  the  liability  of  shocks,  and  if  the  system  is 
grounded  elsewhere,  local  heating  with  the  attendant  loss  of 
power  due  to  stray  currents  may  result. 

Tests  for  Grounds.  In  general,  to  tell  whether  a  machine  is 
grounded,  a  very  simple  test  can  be  made.  A  voltmeter  or  an 
incandescent  lamp  is  placed  in  series  with  a  source  of  E.M.F. 
and  the  free  ends  applied,  one  to  the  frame  or  shaft  of  the  machine 
and  the  other  to  one  of  the  armature  or  field  terminals.  If  the 
voltmeter  deflects  appreciably  or  the  lamp  lights  up,  a  ground 
exists.  The  magnitude  of  the  voltmeter  deflection  and  the 
degree  to  which  the  lamp  lights  up  will  be  an  indication  of  the 
resistance  of  the  ground  circuit,  for  the  greater  the  voltmeter 
deflection  and  the  brighter  the  lamp,  the  less  the  resistance  of 
the  ground  circuit. 

This  test  is  usually  sufficient  for  field  windings,  inasmuch 
as  grounds  can  occur  only  where  the  field  spools  touch  the  iron, 
so  that  the  exact  spot  can  usually  be  located  by  examination  of 
the  spool. 

To  locate  the  exact  coil  in  an  armature  which  is  grounded 
the  apparatus  may  be  arranged  as  in  Fig.  47.  In  this  figure 
A  and  B  are  the  brushes  which  are  connected  by  a  buzzer  S 
or  some  other  device  which  makes  and  breaks  the  current, 
in  series  with  a  battery.  T  is  a  telephone  receiver,  one  end  of 
which  is  permanently  grounded.  G  is  the  ground  which  is  to 
be  located. 

If  the  positive  end  of  the  battery  is  connected  to  brush  A, 
then  the  latter  will  be  at  a  higher  potential  than  B,  and  since  the 
difference  in  potential  between  A  and  B  is  applied  to  one-half 
of  the  winding,  it  follows  that  brush  A  is  at  higher  potential 
than  the  ground  G,  and  that  the  latter  is  similarly  at  higher 
potential  than  brush  B.  In  general  it  is  customary  to  consider 


98  TESTING   OF    ELECTRICAL   MACHINERY 

the  ground  as  being  at  zero  potential,  so  that  it  follows  that  A 
is  above  the  potential  of  the  ground  or  at  positive  potential  and 
B  is  below  the  ground  potential  or  at  negative  potential. 

If  now  the  free  end  of  the  telephone  receiver  T  is  successively 
touched  to  each  commutator  bar  in  the  upper  half  of  the  armature 
in  Fig.  47,  it  will  be  silent  or  nearly  so  only  when  touched  to 
the  particular  bars  to  which  the  grounded  coil  is  connected. 
When  touching  any  other  commutator  bar  the  telephone  receiver 
will  sound,  owing  to  the  pulsating  current  set  up  in  it  by  the 


FIG.  47 

pulsating  difference  of  potential  between  the  bar  touched  and 
the  ground. 

It  will,  however,  be  seen  that  the  same  difference  of  potential 
exists  across  the  lower  half  of  the  winding,  and  since  A  is  above 
and  B  is  below  the  ground  potential,  a  point  will  be  found  in  the 
lower  half  of  the  winding  (Fig.  47)  which  is  at  the  same  potential 
as  the  ground  and  at  which  the  telephone  will  be  silent.  This 
point  is  called  the  phantom  ground,  Ph.  G.  Or  with  the  tele- 
phone receiver  tapped  to  bar  16,  the  difference  in  potential  from 
bar  1 6  to  brush  B  will  be  equal  and  opposite  to  that  from  bar  10 
to  brush  B  and  the  telephone  will  therefore  be  silent. 


LOCATION  OF  FAULTS  IN  A  DIRECT-CURRENT  MOTOR       99 

Two  commutator  bars  have  thus  been  found  at  which  the 
telephone  receiver  is  silent,  one  of  these  being  the  real  and  the 
other  being  the  phantom  ground.  In  order  to  distinguish 
between  them,  the  armature  is  rotated  a  few  degrees  as  in  Fig. 
48.  The  real  ground  must  necessarily  be  found  at  the  same 
place,  but  the  phantom  ground  will  now  be  located  somewhere 
else,  since  the  relative  potential  of  A  and  B  with  respect  to  the 
ground  is  now  different.  In  Fig.  47  the  phantom  ground  is  shown 
between  bars  16  and  17  and  in  Fig.  48  between  bars  i  and  24. 

In  multipolar  machines  the  procedure  is  the  same,  except  that 
the  armature  should  be  converted  to  a  two-circuit  winding.  This 
is  readily  done  by  slipping  paper  between  all  the  brushes  and  the 


FIG.  48 

commutator  and  slipping  the  leads  from  the  battery  between 
the  paper  and  the  commutator  at  two  diametrically  opposite 
sets  of  brushes. 

Buzzer  S  may  be  omitted,  in  which  case  the  free  end  of  the 
telephone  circuit  must  be  tapped  on  the  various  bars.  Each 
time  a  contact  is  made  the  telephone  will  sound.  A  voltmeter 
may  also  be  substituted  for  the  telephone  receiver. 


100  TESTING   OF   ELECTRICAL   MACHINERY 

Method.  First  test  the  faulty  machine  for  open  armature 
coils  by  operating  it  as  a  motor  with  its  shunt  field  separately 
excited  and  its  armature  in  series  with  a  lamp  board.  With  a 
rather  weak  field,  throw  in  enough  lamps  to  make  the  armature 
rotate  slowly.  If  sparking  occurs  and  you  consider  it  due  to 
an  open  coil,  locate  the  bars  at  which  it  occurs  and  verify  by  using 
the  method  outlined,  using  dry  cells  and  a  voltmeter.  If  an  open 
coil  exists,  bridge  the  proper  commutator  bars  after  consulting 
an  instructor.  Then  operate  again  for  one  minute,  stop  and  ex- 
amine the  armature  for  a  heated  coil.  Continue  several  times  and 
make  certain  if  a  short-circuited  turn  exists  by  using  the  volt- 
meter and  dry  cells. 

Finally  test  the  fields  and  armature  for  grounds  by  the  lamp 
or  voltmeter  method  and  if  the  armature  is  found  to  be  grounded 
locate  the  commutator  bars  at  which  it  exists. 

Do  not  rip  off  any  canvas  covering  or  insulation  in  order 
to  find  just  where  the  fault  exists,  unless  specifically  directed. 

Conclusions.  Explain  what  is  meant  by  grounds,  open  and 
short-circuited  armature  coils.  Give  symptoms  and  remedies 
for  them.  What  specific  temporary  repairs  can  be  made  for 
cases  D,  E,  and  F  in  Fig.  46?  Explain  why  a  phantom  ground 
exists  in  Figs.  47  and  48. 


EXPERIMENT  XIV 

The  Direct  Current  Watt-hour  Meter. — In  connection  with 
the  supply  and  sale  of  electrical  energy,  it  is  necessary  to  have 
some  form  of  meter  which  will  record  the  total  amount  of  energy 
used.  Whereas  -power,  the  rate  at  which  energy  is  delivered, 
is  measured  in  watts  or  kilowatts,  energy  is  measured  in  watt- 
hours  or  kilowatt-hours,  the  watt-hour  being  defined  as  the 
total  or  integrated  amount  of  energy  supplied  in  one  hour  to  a 
circuit,  in  which  the  steady  or  average  rate  at  which  energy  is 
expended  is  i  watt.  A  kilowatt-hour  is  then  1000  watt-hours. 

»  Watt-hour  meters  as  they  are  properly  called  are  often 
referred  to  as  "  integrating  watt-meters."  Although  the  meter 
does  integrate  the  amount  of  energy  supplied  to  a  circuit  over  a 
period  of  time,  the  term  "  watt-hour  "  meter  is  preferred,  as  it 
indicates  the  unit  in  which  the  instrument  registers.  The  term 
"  recording  watt-meter,"  as  applied  to  a  watt-hour  meter  is 
incorrect,  as  this  term  really  indicates  a  meter  which  makes  a 
continuous  record  by  means  of  a  pen  or  other  device,  of  the 
instantaneous  watts  on  a  sheet  of  paper,  film,  etc.,  the  same  as  a 
recording  voltmeter  or  ammeter. 

A  watt-hour  meter  consists  essentially  of  (i)  a  small  electric 
motor  so  constructed  that  its  torque  is  proportional  to  the  power 
taken  by  the  load,  (2)  a  brake  system,  so  designed  that  the 
opposing  torque  is  proportional  to  the  speed  of  the  rotating 
shaft  and  (3)  a  system  of  gears  with  numbered  dials  for  registering 
the  number  of  revolutions  of  the  motor  shaft. 

When  the  speed  of  the  rotating  shaft  is  steady  the  driving 
torque  must  be  just  equal  to  the  retarding  torque.  'With  the 
driving  torque  proportional  to  the  power  taken  by  the  load  and 
the  opposing  torque  of  the  brake  system  proportional  to  the  speed 
of  rotation  of  the  shaft,  the  speed  of  the  shaft  is  proportional  to 
the  driving  torque  and  hence  to  the  power.  Therefore  the  total 

101 


102  TESTING   OF   ELECTRICAL   MACHINERY 

number  of  revolutions  which  the  shaft  makes  during  any  interval 
is  proportional  to  the  total  energy  during  this  interval,  whether 
the  power  is  steady  or  variable.  Or,  each  revolution  of  the  shaft 
represents  a  certain  number  of  watt-hours  of  electrical  energy 
having  passed  through  the  meter. 

There  are  two  common  types  of  direct  current  watt-hour 
meters.  The  more  common,  invented  by  Elihu  Thompson,  has 
for  its  driving  element  a  motor  having  fields  and  an  armature 
with  a  small  commutator.  The  second  type  employs  what  is 
known  as  a  mercury  motor. 

In  the  Thompson  or  "  commutating "  type  of  watt-hour 
meter,  the  fields  of  its  driving  motor  are  placed  in  series  with  the 
load  and  therefore  carry  the  load  current.  Obviously  these  will 
be  of  very  low  resistance  and  as  no  iron  is  used  in  the  construction 
of  the  motor,  the  flux  set  up  by  the  field  coils  is  directly  pro- 
portional to  the  load  current.  The  armature  is  placed  directly 
across  the  line  and  its  current,  which  is  reduced  to  a  very  small 
value  by  the  use  of  a  high  resistance,  is  directly  proportional 
to  the  E.M.F.  of  the  supply  line.  Since  the  torque  generated 
by  a  motor  is  proportional  to  the  product  of  field  strength  and 
armature  current,  we  have  in  the  watt-hour  meter,  that  the 
torque  of  its  motor  is  proportional  to  the  power  passing  through 
the  meter. 

The  total  resistance  of  the  armature  circuit  is  about  2500 
ohms  for  no- voltmeters,  about  5000  ohms  for  220  voltmeters, 
etc.  The  resistance  of  the  armature  itself  is  about  1200  ohms 
for  all  voltages,  so  that  about  1300  ohms  resistance  in  series  with 
the  armature  must  be  provided  for  no  volt  watt-hour  meters. 
3800  ohms  for  220  volt  meters,  etc.  Bearing  in  mind  that  no 
iron  is  employed  in  the  construction  of  the  driving  motor,  it  is 
evident,  with  such  high  values  of  armature  resistance  and  the 
very  low  values  of  armature  speeds  used  (25  to  50  R.P.M. 
at  full  load),  that  the  C.E.M.F.  generated  in  the  armature  is 
insignificant,  the  entire  voltage  impressed  being  used  up  as  IR 
drop.  This  can  also  be  shown  by  test;  the  armature  of  a  cer- 
tain 5  ampere,  no  volt  watt-hour  meter  was  found  to  take 


THE   DIRECT   CURRENT   WATT-HOUR   METER  103 

0.045  ampere  with  no  volts  impressed  on  its  armature  circuit 
and  a  load  current  of  5  amperes  was  passing  through  its  fields. 
When  the  armature  was  blocked  so  that  it  could  not  rotate, 
no  change  in  the  armature  current  could  be  observed  on  the 
ammeter. 

The  loss  of  power  in  the  potential  circuit,  from  the  values  of 
resistance  given,  will  be  seen  to  be  about  5  watts  for  no  volt- 
meters, 10  watts  for  220  voltmeters,  etc.  The  loss  of  power  in 
the  current  coils  is  about  5  watts  for  a  5-ampere  meter  and 
increases  with  the  current  capacity  of  the  meter. 

The  brake  system  of  all  modern  types  of  watt-hour  meters 
consists  of  a  disc  of  aluminum  mounted  on  the  armature  spindle. 
One  or  more  permanent  magnets  are  so  arranged  that  the  disc 
rotates  between  their  poles.  As  the  disc  rotates,  eddy  currents 
are  generated  in  the  disc,  producing  a  drag  on  the  disc.  The 
strength  of  the  eddy  currents  generated  depends  directly 
upon  the  speed  of  the  disc,  since  the  strength  of  the  permanent 
magnets  is  constant,  so  that  the  force  between  the  eddy  currents 
and  the  field  of  the  magnets,  and  hence  the  drag  on  the  disc,  is 
directly  proportional  to  the  speed. 

In  Fig.  49  is  shown  the  general  form  of  a  direct  current  watt- 
hour  meter  of  the  commutator  type.  The  armature  A ,  made  in 
spherical  form,  is  mounted  on  a  vertical  steel  spindle  which  is 
supported  at  the  top  by  a  guide  bearing  and  at  the  bottom  by 
either  a  pivot  and  jewel,  or  ball  and  jewel,  bearing.  In  the  first 
type  of  lower  bearing,  the  spindle  terminates  in  a  pivot  which  is 
supported  by  a  jewel,  while  in  the  latter  a  jewel  attached  to  the 
lower  end  of  the  spindle  rests  on  a  steel  ball  which  in  turn  is 
supported  by  a  second  jewel.  The  jewels  used  are  selected 
sapphires  and  diamonds.  The  main  fields,  fixed  in  position,  are 
designated  FF  and  the  disc  D  is  shown  at  the  bottom,  rotating 
between  the  poles  of  the  magnets  MM.  The  potential  circuit, 
shown  in  broken  lines,  starts  at  terminal  x  passes  successively 
through  the  compensating  field  C  (taken  up  later) ,  the  armature, 
the  fixed  resistance  R  and  ends  at  terminal  y.  In  some  meters 
all  the  necessary  re^'ztance  in  series  with  the  armature  is  con- 


104 


TESTING   OF   ELECTRICAL   MACHINERY 


tained  in  the  compensating  field  and  no  extra  fixed  resistance  R 
is  necessary. 

It  is  evident  from  the  preceding  description  that  a  certain 
amount  of  friction,  due  to  the  brushes  on  the  commutator,  the 
gear  train,  and  the  bearings,  is  present  and  approximately  inde- 
pendent of  any  variations  of  the  load.  Brush  friction,  which  is 


Line 


Load 


FIG.  49 

more  or  less  variable  with  time,  is  by  far  the  most  important. 
At  light  loads  friction  represents  a  larger  percentage  of  the  total 
driving  torque  than  at  heavy  loads.  It  is  thus  desirable  that 
the  torque  exerted  by  the  driving  motor  be  as  high  as  practi- 
cable in  order  to  reduce  the  percentage  of  driving  torque  neces- 
sary to  overcome  any  change  in  friction.  This  results  in  a 
so-called  "  high  torque  "  meter. 

In  order  to  reduce  friction  to  a  minimum,  the  commutator 
is  made  extremely  small  (about  one-tenth  inch  in  diameter)  and 
in  order  that  its  surface  remain  smooth  it  is  generally  made  of 
pure  silver  and  the  brushes  silver- tipped  to  prevent  oxidation. 


THE   DIRECT   CURRENT   WATT-HOUR   METER  105 

To  compensate  for  friction,  all  watt-hour  meters  have  a  com- 
pensating device,  connected  across  the  line  so  that  the  excitation 
is  independent  of  the  load  on  the  meter.  In  the  direct  current 
commutator  watt-hour  meter,  it  consists  of  an  auxiliary  field 
coil,  capable  of  being  moved  toward  or  away  from  the  armature 
and  clamped  in  the  correct  position.  The  strength  of  the  com- 
pensating field  is  constant,  but  by  varying  its  position  with 
respect  to  the  armature,  enough  of  its  flux  passes  through  the 
armature  to  create  a  torque  which  will  just  balance  friction,  so 
that  the  meter  is  just  on  the  point  of  starting  at  no  load.  Evi- 
dently if  the  compensating  field  is  too  close  to  the  armature,  more 
torque  than  necessary  will  result  and  the  meter  will  " creep" 
or  rotate  slowly  without  any  load  current  through  its  main 
fields,  and  for  loads  below  10  per  cent,  it  will  register  "  fast "  or 
too  high. 

At  higher  loads  the  effect  of  the  compensating  field  becomes 
more  and  more  negligible  compared  to  the  main  field  and  in  order 
to  control  the  speed  of  the  disc  under  such  loads,  the  position  of 
the  permanent  magnets  is  changed.  If  they  are  moved  in 
toward  the  center  of  the  disc,  the  speed  of  the  disc  will  increase 
for  a  given  load,  in  order  to  maintain  the  same  rate  of  cutting  of 
the  flux  from  the  permanent  magnets.  If  the  magnets  are 
moved  out  from  the  center  of  the  disc,  the  reverse  is  true. 

Several  other  factors  affect  the  accuracy  of  watt-hour  meters 
and  their  permanency  of  calibration.  Variation  of  voltage  will 
slightly  affect  the  strength  of  the  compensating  field.  Due 
to  overloads  and  short-circuits,  the  magnetization  of  the  per- 
manents  magnets  is  apt  to  be  diminished,  resulting  in  a  fast 
meter.  Wear  of  the  bearings,  brushes,  commutator,  etc.,  will 
cause  the  friction  in  a  meter  to  change  with  time  and  vibration, 
vermin  and  possible  corrosion  due  to  dampness  and  fumes,  are 
apt  to  interfere  with  its  operation.  It  therefore  becomes  advis- 
able to  periodically  inspect  and  test  meters  in  order  to  keep 
them  in  proper  condition.  For  meters  in  ordinary  service,  one  or 
two  routine  tests  per  year  is  sufficient. 

All  modern  watt-hour  meters  register  in  kilowatt-hours.     In 


106  TESTING   OF   ELECTRICAL   MACHINERY 

smaller  sizes  this  reading  is  determined  directly  from  the  dials, 
while  in  the  larger  sizes  a  "dial"  constant,  usually  some  multiple 
of  ten,  must  be  applied  to  the  dial  reading  to  get  kilowatt-hours. 
Dial  constants  are  always  clearly  indicated. 

In  testing  watt-hours  the  two  common  methods  used  are 
either  a  voltmeter  and  ammeter  or  a  rotating  standard  watt- 
hour  meter.  In  the  former  method  a  certain  amount  of  power 
from  a  constant  source  of  voltage  is  passed  through  the  meter 
to  be  tested  for  a  definite  time  and  the  number  of  revolutions  of 
the  meter  counted  in  that  time.  The  power  is  obtained  from 
voltmeter  and  ammeter  readings,  the  instruments  being  con- 
nected as  in  Fig.  49,  and  the  time  is  determined  by  means  of  a 
stop-watch. 

Since  power  is  equal  to  watt-seconds  per  second  or  watt-hours 
times  3600  divided  by  seconds,  we  have  that  the  average  power 
as  registered  by  the  watt-hour  meter  is 

.......     (19) 


where  R  is  the  number  of  revolutions  of  the  disc  in  /  seconds  and 
K,  the  meter  "  disc  "  constant,  represents  the  watt-hours  per 
revolution  of  the  meter.  In  many  types  of  meters,  the  disc 
constant  is  marked  on  the  disc  itself  and  usually  expressed  as 
above  in  watt-hours  per  revolution.  Sometimes  the  constant  is 
given  in  watt-seconds  per  revolution,  in  which  case  the  term 
3600  in  the  expression  above  is  not  necessary.  In  some  direct 
current  watt-hour  meters  made  by  the  Westinghouse  Electric 
and  Manufacturing  Co.,  the  disc  constant,  expressed  in  watt- 
seconds  per  revolution,  is  obtained  by  multiplying  the  product 
of  the  rated,  voltage  and  current  of  the  meter  by  2.4. 

The  accuracy  of  the  watt-hour  meter  under  test  is  then  the 
ratio  (usually  expressed  in  per  cent)  of  the  power  as  registered 
by  the  meter  to  the  power  as  measured  by  the  standard  ammeter 
and  voltmeter. 

Whenever  the  load  or  voltage  fluctuations  are  such  as  to 
make  it  difficult  to  average  the  ammeter  and  voltmeter  readings 


THE   DIRECT    CURRENT   WATT-HOUR   METER  107 

or  if  only  one  tester  is  available,  the  ammeter-voltmeter  method 
of  testing  is  not  used,  a  portable  standard  rotating  watt-hour 
meter  being  substituted,  In  such  a  standard  a  finger,  rotating 
over  a  dial,  is  attached  directly  to  the  spindle,  the  dial  being 
readable  to  o.oi  revolution.  With  the  current  coils  of  the 
standard  in  series  and  its  potential  coils  in  parallel  with  those  of 
the  meter  under  test,  it  is  only  necessary  to  compare  the  number 
of  revolutions  made  by  the  standard  during  a  given  time  with 
the  number  made  by  the  meter  under  test.  The  average  powers 
indicated  by  the  meter  under  test  (/)  and  the  standard  (s)  are 

and     Ps  =  A-s; 


and  the  accuracy  of  the  meter  under  test 
Ptioo     K,Ntioo 


Ps  KSNS 


(20) 


In  order  that  the  disc  constants  of  both  meters  be  the  same 
as  often  as  possible,  standard  watt-hour  meters  are  made  with 
a  number  of  different  current  and  potential  capacities. 

Standard  meters  are  started  and  stopped  by  either  making 
and  breaking  the  potential  circuit  or  else  throwing  the  register 
in  and  out  of  gear  by  an  electrically  operated  clutch. 

In  the  mercury  flotation  type  of  watt-hour  meter,  advantage 
is  taken  of  the  principle  that  when  a  pivoted  metallic  disc  carry- 
ing current  is  placed  under  the  influence  of  a  magnetic  field,  the 
disc  will  rotate. 

Fig.  50  shows  the  essentials  of  a  direct  current  watc-hour 
meter  of  this  type.  A  copper  armature  disc  D,  slotted  radiallly 
to  guide  the  current  and  supported  on  a  spindle  S,  is  enclosed 
in  a  suitable  chamber  B7  made  of  moulded  insulating  material 
partially  filled  with  mercury.  Current  is  led  in  and  out  of  the 
mercury  chamber  by  means  of  the  copper  lugs  L  which  are  set 
diameterically  opposite  each  other.  The  current  passes  from  one 
lug  through  the  comparatively  high-resistance  mercury  to  the 
edge  of  the  low-resistance  copper  disc,  across  the  disc  to  the  mer- 


108 


TESTING   OF   ELECTRICAL   MACHINERY 


cury  and  out  from  the  other  lug.  The  field  magnet  Y  is  built 
up  of  steel  laminations  and  carries  the  two  windings  SW  and  CW. 
The  magnetic  circuit  is  completed  by  the  circular  ring  Z  made  up 
of  steel  ribbon  and  placed  over  the  mercury  chamber. 


FIG.  50 

At  the  top  of  Fig.  50  is  shown  the  damping  device  which  is  the 
same  as  described  before.  By  attaching  a  small  float  F  of  hard 
wood  on  top  of  the  armature  disc  B,  the  entire  moving  system  is 
given  just  a  little  excess  buoyancy  so  as  to  exert  a  slight  upward 
pressure  against  a  jewel  bearing  /  at  the  top.  the  spindle  being 
kept  in  alignment  by  a  guide  bearing  G  at  the  bottom.  Bearing 
friction  is  thus  very  much  reduced. 

The  circuits  of  the  meter  are  easy  to  follow.     The  potential 


THE   DIRECT   CURRENT   WATT-HOUR   METER  109 

circuit,  taken  from  the  line  terminals,  in  most  mercury  watt-hour 
meters  passes  through  a  small  heating  unit  of  a  thermo-couple 
and  then  around  the  field  magnets.  The  current  circuit  is  given 
one  turn  CW  around  each  leg  of  the  field  magnets,  in  order  to 
overcome  the  fluid  friction  of  the  mercury  which  increases  with 
the  speed  of  the  armature  disc.  A  compounding  action  with 
load  thus  results. 

The  light  load  adjustment  in  most  mercury  flotation  watt- 
hour  meters  is  obtained  by  the  use  of  a  thermo-couple  H  whose 
heating  element  is  in  series  with  the  potential  circuit.  The  two 
dissimilar  metals  of  the  thermo-couple  generate  a  voltage 
across  the  points  M  and  P,  which  is  constant  so  long  as  the  tem- 
perature of  the  heating  element  and  therefore  the  line  voltage  is 
constant.  Two  rods  R  and  C,  joined  electrically  by  a  slider  K, 
are  used  to  make  the  necessary  adjustment.  Rod  R  is  of  resist- 
ance material  and  rod  C  is  of  copper.  The  current  generated 
by  the  thermo-couple  divides  at  M.  part  flowing  through  rod 
R  to  K,  returning  over  rod  C  to  P.  The  other  portion  of  the 
current  flows  through  the  armature  disc  in  the  same  direction  as 
the  load  current,  providing  the  necessary  torque  to  overcome 
friction.  By  moving  slider  K  to  the  right  more  of  the  resistance 
rod  is  put  into  circuit  and  more  current  flows  through  the  arma- 
ture disc,  giving  greater  starting  torque. 

Another  method  of  making  the  light  load  adjustment  that 
is  sometimes  used,  is  to  shunt  some  of  the  potential  circuit 
current  through  the  armature  disc,  the  final  adjustment  being 
made  by  the  use  of  two  rods  in  a  manner  similar  to  that  used  in 
the  thermo-couple  method. 

In  most  mercury  watt-hour  meters  the  position  of  the  per- 
manent magnets  is  fixed  and  in  order  to  adjust  the  action  of  the 
retarding  disc,  magnetic  shunts  in  proximity  to  the  magnets  are 
provided.  By  moving  these  closer  to  or  farther  from  the  mag- 
nets, more  or  less  of  the  flux  from  the  magnets  is  shunted  away 
from  the  retarding  disc. 

The  full-load  drop  through  the  armature  disc  of  mercury 
watt-hour  meters  is  so  low  that  it  is  customary  to  build 
one  standard  meter  for  10  amperes  and  to  use  external  shunts 


110 


TESTING   OF   ELECTRICAL   MACHINERY 


for  larger  sizes.  As  a  result  the  energy  lost  through  the  arma- 
ture disc  is  comparatively  low.  The  resistance  and  energy 
losses  of  the  potential  circuits  is  about  the  same  as  in  com- 
mutator types.  For  meters  below  130  volts  the  entire  resistance 
of  the  potential  circuit  is  in  the  field  winding,  extra  resistance 
in  series  being  added  for  higher  voltages. 

Connect  the  meter  to  be  tested  as  shown  in  the  text,  using 
some  suitable  load.  After  applying  potential,  load  circuit  being 
open,  see  whether  the  meter  has  any  creep.  If  it  has,  determine 
its  rate.  Then  leaving  the  meter  as  found,  determine  its  accu- 
racy at  10,  50  and  100  per  cent  load,  taking  the  time  of  a  con- 
venient number  of  revolutions  for  a  period  of  about  one  minute. 
Take  enough  sets  of  readings  for  each  load  so  that  the  per  cent 
registration  of  three  sets  check  within  one  per  cent.  Then 
reduce  the  load  to  10  per  cent  and  adjust  the  meter  to  within 
2  per  cent  accuracy  and  do  the  same  for  100  per  cent  load. 
After  noting  that  the  adjustment  at  light  load  is  still  within 
2  per  cent,  determine  the  accuracy  of  the  meter  for  loads  of  10, 
25,  50,  75,  ico,  125  and  150  per  cent  loads.  Record  all  readings 
as  in  Table  XII. 


TABLE   XII 


.  volts 


Make  and  type  of  meter amperes 

Maker's  meter  number 

Rated  full  load  on  meter  during  tests watts     L. 

Rate    of  creep revolutions    in minutes seconds. 

Disc  constant  .fiT Testing  constant  Kt  =  KX^oo 


Volts. 

Amperes. 

True 
Watts 
A  XB=C 

Rated 
Load 

=  I00y-. 

Meter  under  Test. 

Ob- 
served. 

True. 
A 

Ob- 
served. 

True. 

Revs. 

Sees. 

Watts 
w  =  KtXR 

regis- 
tration 
W 
=  IOCT 

THE   DIRECT   CURRENT  WATT-HOUR   METER  111 

Curves.  Plot  a  curve  between  per  cent  registration  (ordinate) 
and  per  cent  load  from  the  results  obtained  after  the  meter  was 
adjusted  at  light  and  full  loads. 

Conclusions.  What  characteristics  should  a  watt-hour  meter 
possess?  Why  are  the  light  load  and  full  load  adjustments 
necessary?  Which  is  the  more  important?  Why?  What 
would  be  the  probable  effect  of  dust,  fumes,  and  vermin  getting 
into  a  meter  and  what  sort  of  a  cover  should  a  watt-hour  meter 
therefore  have?  What  appear  to  you  to  be  the  advantages  and 
disadvantages  of  the  two  types  of  watt-hour  meters  described 
in  the  text?  As  the  meters  are  connected  in  Figs.  49  and  50 
and  disregarding  the  accuracy  of  the  meters,  who  is  charged 
with  the  losses  in  the  potential  and  current  circuits  of  the  meters? 
If  the  meter  had  any  creep  at  the  start  of  the  test,  calculate 
what  it  would  cost  the  customer  per  day  at  ten  cents  per  kilo- 
watt-hour. 


EXPERIMENT  XV 

The  Lead  Storage  Battery. — Storage  batteries  are  classified 
according  to  their  electrolytes,  as  "acid"  or  " alkaline."  The 
former  include  all  of  the  so-called  lead  cells  and  the  latter  is 
represented  by  one  commercial  type,  the  "iron-nickel"  or  Edison 
cell.  This  experiment  is  intended  to  give  the  student  some 
knowledge  of  the  handling  of  a  lead  cell  and  understanding  of 
what  occurs  within  a  cell  as  it  is  put  through  a  cycle  of  discharge 
and  charge. 

A  storage  battery  when  charged  comprises  lead  grid  positive 
plates  carrying  active  material  of  lead  peroxide  (dark  brown  in 
color)  and  lead  grid  negative  plates  with  active  material  of  sponge 
lead  (dark  slate  in  color)  and  an  electrolyte  of  sulphuric  acid 
varying  in  density  from  1.23  to  1.3  depending  upon  its  purpose. 

Neither  sponge  lead  nor  lead  peroxide  possesses  strength, 
rigidity  or  high  conductivity.  Mechanical  considerations  require 
the  first  two  and  the  latter  is  necessary  to  conduct  the  current 
away  efficiently.  Therefore  the  finely  divided  and  porous  lead 
and  lead  peroxide  are  held  in  suitable  grids  which  are  a  rigid 
framework  or  plate  of  lead  or  an  alloy  of  lead  and  antimony. 

When  the  electrodes  are  in  condition  to  furnish  current, 
the  battery  is  said  to  be  charged.  The  process  of  giving  out 
current  is  called  discharge,  during  which  some  of  the  active 
material  of  both  plates  changes  to  lead  sulphate,  the  plates 
becoming  lighter  in  color.  The  electro-motive  force  of  the 
battery  also  gradually  falls  and  the  density  of  the  electrolyte 
decreases.  After  the  battery  has  furnished  such  an  amount  of 
energy  as  to  bring  its  E.M.F.  down  to  a  predetermined  value, 
the  battery  is  said  to  be  discharged.  Charging  is  the  process 
whereby  current  is  sent  through  the  cell  from  an  outside  source 
in  a  direction  opposite  to  the  flow  on  discharge,  thereby  changing 

112 


THE  LEAD  STORAGE  BATTERY  113 

the  exhausted  active  material  back  to  its  original  useful  condi- 
tion. 

The  chemical  changes  taking  place  during  discharge  may  be 
represented  by  the  following  equations.  When  read  from  left  to 
right  they  represent  discharge  and  from  right  to  left,  the  reac- 
tions of  charging  are  shown.  The  reactions  at  the  positive  plate 
are, 


=  PbSO4+H2O+O,     .     .     .     (21) 
and  at  the  negative  plate, 

Pb+H2S04  =  PbS04+H2.       .     .     .     (22) 
Combining  we  may  write  for  the  whole  cell, 

PbO2+Pb  +  2H2SO4  =  2PbSO4+H2O.       .     .     (23) 

The  capacity  of  a  storage  battery  is  stated  either  in  ampere- 
hours  or  watt-hours.  The  latter  is  naturally  more  important 
as  it  takes  into  account  not  only  the  current  capacity  but  also 
the  cell  voltage.  The  ampere-hour  capacity  of  a  cell  can  usually 
be  approximated  by  assuming  that  from  20  to  25  square  inches 
of  positive  plate  surface  (both  sides),  with  plates  f  inch  thick 
will  give  i  ampere  for  eight  hours  or  eight  ampere-hours  at  an 
eight-hour  rate  of  discharge.  For  other  thicknesses,  the  capacity 
varies  as  the  square  root  of  the  plate  thickness.  Although  the 
theoretical  weights  of  lead  peroxide  and  of  sponge  lead  are 
respectively  0.156  and  0.137  ounce  per  ampere-hour  capacity, 
in  practice  it  is  usual  to  allow  from  2.5  to  3  times  these  weights. 
This  is  done  to  allow  for  the  lack  of  porosity  and  gradual  scaling 
and  shedding  of  the  active  materials.  Since  the  negative  plates 
do  not  retain  their  capacity  as  well  as  the  positives,  there  is 
generally  one  more  negative  than  positive  plates  in  a  battery. 

From  the  equations  given  it  is  evident  that  on  discharge  the 
density  of  the  electrolyte  will  fall  and  lead  sulphate  be  formed. 
Pure  lead  sulphate,  when  isolated,  is  white  in  color,  has  such  high 
resistance  as  to  be  practically  an  insulator  and  has  greater  volume 
than  that  of  the  lead  or  lead  peroxide  from  which  it  is  formed. 


114  TESTING   OF   ELECTRICAL   MACHINERY 

If  an  excessive  quantity  of  lead  sulphate  is  allowed  to  form  by 
discharging  a  cell  beyond  some  safe  limit,  the  plates  may  be 
injured.  The  excessive  volume  of  sulphate  will  tend  to  cause 
warping  and  cracking  of  the  plates  and  loosening  of  the  active 
material.  Lead  sulphate  which  forms  on  the  plates  during  dis- 
charge is  readily  reducible  by  current,  so  that,  if  soon  after  a 
cell  is  discharged  it  is  put  on  charge,  no  trouble  results.  But 
if  a  discharged  cell  is  allowed  to  stand  idle  for  any  time,  the 
sulphate  apparently  changes  its  nature,  becoming  dense  and 
inactive,  so  that  recharging  will  not  readily  reduce  it.  The 
formation  of  this  insoluble  sulphate  is  called  sulphation  and  if 
it  has  not  gone  too  far,  the  plates  may  be  cleared  by  overcharging 
the  battery  for  a  long  time.  When  a  battery  is  fully  charged 
further  passage  of  current  will  cause  electrolysis  of  the  water 
and  the  cell  is  said  to  be  gassing.  The  liberation  of  oxygen 
and  hydrogen  thus  formed  tends  to  tear  off  the  insoluble 
sulphate. 

The  open  circuit  E.M.F.  of  a  cell  falls  somewhat  with 
decrease  of  density  of  the  electrolyte  and  temperature  and  is 
also  dependent  upon  the  character  of  the  active  material.  When 
fully  charged  the  open-circuit  voltage  of  a  cell  is  about  2.20 
volts.  While  being  discharged  the  voltage  falls  more  or  less 
rapidly  during  the  first  fifteen  minutes,  then  much  more  slowly 
until  the  cell  is  nearly  discharged,  when  it  again  falls  rapidly. 
The  decrease  in  voltage  is  largely  caused  by  the  formation  of 
lead  sulphate  and  increase  in  resistance.  At  first  the  chemical 
action  takes  place  largely  on  the  surface  of  the  plates,  the  re- 
sulting formation  of  sulphate  tending  to  close  up  the  pores  in 
the  active  material.  As  discharge  continues  the  electrolyte 
occluded  in  the  pores  gives  up  its  constituents,  becoming  water, 
and  as  new  electrolyte  cannot  diffuse  into  the  pores  the  cell 
becomes  discharged.  However,  on  standing,  as  electrolyte  slowly 
works  its  way  into  the  pores,  the  battery  recovers  somewhat 
arid  will  have  some  residual  charge. 

From  what  has  been  said  it  is  evident  that  the  ampere-hour 
capacity  of  a  cell  will  decrease  as  the  rate  of  discharge  is  increased. 


THE  LEAD  STORAGE  BATTERY  115 

Approximate  values  of  capacity  for  different  rates  of  discharge 
are  given  in  the  following  table: 

Hours  Rate  Ampere-hour  Capacity  in  Per  Cent 

ot  Discharge.  of  Eight  Hour  Rate  Capacity. 

1  55 

2  70 

3  80 

4  88 

5  93 
8                                         100 

12  no 

From  the  foregoing  table  it  is  necessary  to  adopt  some 
standard  rate  to  fix  the  capacity  of  a  cell  and  this  is  usually 
taken  as  the  eight-hour  rate.  Thus  an  80  ampere-hour  battery 
is  one  which  will  furnish  10  amperes  continuously  for  eight  hours. 
However,  practice  demands  in  many  cases  rates  of  discharge 
different  from  the  eight-hour  rate  and  the  capacity  of  batteries 
is  often  stated  at  other  rates. 

The  limiting  terminal  voltage  to  which  a  cell  may  safely  be 
allowed  to  fall  on  discharge  also  varies  with  the  rate,  being  lower 
at  high  rates  than  on  low  ones.  In  the  absence  of  data  from  the 
manufacturer  the  limiting  terminal  voltage  (corresponding  to 
the  T  hour  rate  of  discharge)  may  be  taken  from  the  expres- 
sion, 

£  =  1.56+0.01757, (24) 

where  T  =  hours  rate  of  discharge  as  in  the  above  table. 

Every  cell  has  internal  resistance  which  is  also  a  variable, 
causing  an  internal  IR  drop  within  the  cell  upon  the  passage  of 
current.  On  discharge  this  subtracts  from  the  E.M.F.  of  the 
cell,  causing  a  lower  terminal  or  closed  circuit  voltage  and  the 
reverse  on  charge.  The  resistance  of  a  cell  is  increased  by  polar- 
ization on  charge  and  with  the  formation  of  lead  sulphate  on  dis- 
charge. 

The  density  of  the  electrolyte  in  a  fully  charged  battery 
should  be  from  1.23  to  1.3  and  when  the  cell  is  fully  discharged 
should  have  fallen  about  o.  i  in  specific  gravity. 


116 


TESTING   OF   ELECTRICAL   MACHINERY 


Whereas  a  battery  may  be  discharged  at  high  rates,  it  is  not 
advisable  to  charge  it  too  rapidly.  Good  practice  starts  charg- 
ing at  a  five-  or  eight-hour  rate  which  is  tapered  off  as  the  cell 
approaches  full  charge  and  starts  to  gas.  As  the  time  allowed 
for  laboratory  work  is  generally  much  less  than  this,  the  battery 
may  be  charged  at  higher  rates.  It  must,  however,  be  remem- 
bered that  this  is  not  the  best  practice.  During  charging  the 
E.M.F.  of  a  cell  rises  rapidly  at  first,  then  remains  nearly  con- 
stant for  a  period  and  again  rises  rapidly  where  the  battery 


FIG.  51 


approaches  full  charge.  The  curve  is  thus  the  opposite  of  that 
on  discharge. 

The  cell  connections  for  battery  tests  are  as  given  in  Fig.  51. 
With  the  switch  thrown  to  the  left  the  cell  can  be  discharged 
through  the  adjustable  resistance  D.  To  charge  the  cell  the 
switch  is  thrown  to  the  right  and  the  rate  of  charge  controlled 
by  means  of  the  lamp  board  and  the  adjustable  resistance  C. 

Two  batteries  are  to  be  tested  in  this  experiment,  one  under 
charge  and  another  under  discharge.  The  battery  to  be  charged 
is  connected  as  in  Fig.  51,  using  a  carbon  rheostat  and  a  lamp 
board  in  series  with  the  no-volt  line.  Charge  the  battery  at 
about  a  two  and  one-half  hour  rate,  maintaining  this  rate  constant 
by  means  of  the  adjustable  rheostat  C.  Readings  of  open-  and 
closed -circuit  voltage,  specific  gravity  by  hydrometer  and  tem- 
perature are  to  be  taken  every  three  minutes  for  the  first  fifteen 
minutes  or  until  conditions  steady  down.  After  that,  readings 
may  be  taken  at  longer  intervals  until  near  the  end  of  the  run 
when,  conditions  again  changing  rapidly,  three-minute  readings 
must  be  taken.  Continue  charging  until  the  terminal  voltage 
and  specific  gravity  remain  constant  for  several  readings. 


THE  LEAD  STORAGE  BATTERY  117 

After  the  battery  to  be  charged  has  been  charging  for  ten  or 
fifteen  minutes,  start  the  second  cell  on  discharge  at  the  two- 
hour  rate,  maintaining  this  rate  by  means  of  a  carbon  rheostat. 
The  same  readings  are  to  be  taken  at  similar  intervals  as  above. 
Continue  discharge  until  the  open-circuit  voltage  falls  to  a  value 
as  calculated  from  Eq.  (24). 

Curves.  Plot  curves  for  each  cell  on  separate  sheets,  plotting 
elapsed  time  values  as  abscissas  against  open-  and  closed-circuit 
volts,  specific  gravity,  temperature  and  resistance.  The  latter 
is  determined  by  dividing  the  difference  between  open-  and 
closed-circuit  voltages  by  the  current. 

Conclusions.  Explain  why  the  curves  come  out  as  they  do. 
Why  does  the  terminal  voltage  of  a  cell  fall  on  discharge?  Why 
should  a  lead  battery  not  be  completely  discharged?  What 
factors  influence  the  life  of  a  battery?  A  battery  is  to  be  made 
up  of  cells  such  as  you  tested  to  furnish  100  amperes  at  125  volts. 
How  many  cells  would  be  necessary  and  howr  would  you  arrange 
them? 


EXPERIMENT  XVI 

Illumination  Laws  and  Measurements. — The  unit  of  intensity 
of  a  light  source  is  the  candle-power,  and  represents  a  measure 
of  the  light  flux  or  luminous  energy  radiated  by  the  source. 
Formerly,  this  unit  was  defined  as  the  horizontal  intensity  of 
light  emitted  from  a  specified  candle  (British  standard  candle) 
but  due  to  the  greater  permanence  and  reproduceability  of  the 
incandescent  lamp,  the  standard  is  now  maintained  in  tested 
incandescent  lamps  at  the  Bureau  of  Standards,  Washington,  as 
well  as  at  other  laboratories.  The  international  candle  (the 
standard  in  Great  Britain,  France,  and  the  United  States)  is  in 
general  use  to-day,  and  is  1.6  per  cent  less  than  the  British  candle 
referred  to  above. 

The  unit  of  illumination  is  the  foot-candle,  and  is  denned  as 
the  illumination  on  a  plane  i  foot  distant  from  a  source  of 


Illumination  =  1  ft.  candle 

=  1  Jumen/sq.ft. 


FIG.  52 

light  having  an  intensity  equal  to  i  candle-power,  the  plane 
being  normal  to  the  direction  of  the  incident  light  flux  at  all 
points.  Thus,  considering  a  sphere  of  i  foot  radius  (Fig.  52) 

118 


ILLUMINATION   LAWS   AND   MEASUREMENTS  119 

with  a  light  source  placed  at  its  center  having  an  intensity  of  i 
candle-power  in  all  directions,  the  illumination  at  all  points  on  the 
surrounding  sphere  will  be  i  foot-candle. 

In  discussing  illumination,  it  is  helpful  and  convenient  to 
consider  the  luminous  energy  radiation  as  a  flux  emanating  from 
the  light  source.  The  amount  of  such  flux  is  expressed  in  lumens, 
an  illumination  of  i  foot-candle  being  produced  by  i  lumen  per 
square  foot,  or  • 

i  foot-candle  =  i  lumen  per  square  foot. 

Referring  to  Fig.  52,  it  is  evident  that,  since  the  area  of  the 
surrounding  sphere  is  ^r2  or  4?r  square  feet,  and  the  illumina- 
tion produced  is  i  foot-candle  or  i  lumen  per  square  foot,  the 
total  flux  emitted  is  4?r  lumens.  It  follows  that 

Light  flux  (in  lumens)  =  4?rCP     .     .     .     .     (25) 

where  CP  represents  the  mean  spherical  candle  power  of  the 
source. 

Similarly,  on  a  sphere  of  radius  2r  or  2  feet,  the  illumination 
would  equal 

or     —     (in  ft. -candles  or  lumens  per  square  foot). 

47r(2)2  4 

From  the  above,  it  may  be  seen  that 

CP 

Illumination  (on  plane  normal  to  incident  light  flux)  =—        (26) 

a2 

(where  dis  in  feet). 

This  is  known  as  the  "  inverse  square  "  law,  and  holds  for 
all  cases  where  the  maximum  dimension  of  the  light  sources  does 
not  exceed  o.id,  the  distance  between  the  source  and  the  illumi- 
nated plane. 

In  case  the  illuminated  plane  is  not  normal  to  the  incident 
light  (Fig.  53),  it  may  be  readily  shown  that: 

CP 

Illumination  on  horizontal  plane  —  j^  cos3  a.     .     .     (27) 

This  relation  is  known  as  the  "  cosine  "  law,  or  Lambert's 
law,  and  it  is  of  great  use  in  calculating  floor  illumination,  etc. 


120  TESTING   OF   ELECTRICAL   MACHINERY 

The  light  radiated  by  incandescent  lamps  is  far  from  being 
uniform  in  all  directions,  as  shown  by  the  distribution  curves  of  a 
standard  type  of  lamp  (Fig.  54). 

Source  having  a 

light  intensity  =  CP. 
along  line  a-b 


/-Illumination  =-£§  cos3 a 
/ H* 

FIG.  53 

The  candle-power  of  an  incandescent  lamp  has,  therefore, 
no  meaning  unless  it  is  clearly  denned,  thus:  mean  horizontal 
candle-power  (M.H.C.P.)  represents  the  average  intensity  in  the 
horizontal  plane  (Fig.  54^);  mean  spherical  candle-power 


(a)  Horizontal  Distribution.  (b)  Vertical  Distribution 

FIG.  54 

(M.S.C.P.)  represents  the  average  intensity  in  all  directions, 
and  is  the  value  which  must  be  used  in  Eq.  (25).  Mean  upper 
hemispherical  candle-power  (M.U.H.C.P.)  and  mean  lower 
hemispherical  candle-power  (M.L.H.C.P.)  are  also  used.  Where 
the  mean  hemispherical  candle-power  is  discussed,  without  spe- 
cifying which  hemisphere  is  referred  to,  the  lower  is  usually  the 
one  considered. 

The   M.H.C.P.   is  readily  determined,   whereas  the  deter- 
mination of  M.S.C.P.  requires  a  rather  lengthy  investigation 


ILLUMINATION   LAWS   AND   MEASUREMENTS  121 

unless  an  integrating  sphere  is  used.     The  two  values  are  related 
by  the  "  reduction  factor,"  thus 

Reduction  Factor  =     '   '    '    ' (28) 

JM.ri.C.r . 

Therefore,  with  the  reduction  factor  known  for  a  certain 
type  of  lamp,  the  measurement  of  M.H.C.P.  also  permits  the 
determination  of  M.S.C.P. 

The  integrating  sphere  is  a  convenient  device  which  may 
be  used  to  determine  the  M.S.C.P.  If  a  light  source  is  placed  at 
the  center  of  a  hollow  sphere,  the  illumination,  due  to  reflected 
light,  on  the  interior  surface  is  directly  proportional  to  the  total 
light  flux  emitted  by  the  source.  By  measuring  the  illumination 
on  this  surface,  a  measure  of  the  total  flux  (and  thus  M.S.C.P.) 
is  obtained.  A  screen  is  interposed  between  the  light  source  and 
the  point  of  measurement  to  eliminate  the  effect  of  direct  illumina- 
tion. The  sphere  must  first  be  calibrated,  using  a  light  source 
whose  M.S.C.P.  is  known. 

The  determination  of  illumination  requires  an  instrument 
of  light,  rugged  construction  which  may  be  readily  carried  by 
the  observer  to  the  different  locations  where  measurements  are 
required.  The  Macbeth  Illuminometer,  the  General  Electric 
Foot-candle  meter,  and  the  Sharp-Millar  photometer,  fulfill  this 
requirement,  and  are  representative  of  this  type  of  instrument. 
Only  the  first  two  mentioned  will  be  considered  in  this  study. 

(a)  Macbeth  Illuminometer. — The  instrument  is  comprised 
of  three  elements,  as  shown  in  Fig.  55,  namely,  the  illuminometer, 
controller,  and  primary  standard. 

The  operation  of  the  instrument  is  briefly  as  follows:  the 
working  standard  in  the  illuminometer  is  first  calibrated  against 
the  primary  standard  by  placing  the  latter  on  the  test  plate  (a 
white  diffusing  surface)  and  sighting  with  the  illuminometer  into 
the  aperture  D.  The  current  through  the  primary  standard 
is  then  adjusted  to  the  value  specified  in  the  certificate  furnished 
with  the  instrument  to  give  3  foot-candles;  also  the  rachet 
illuminometer  bar  is  set  to  a  scale  reading  of  3  foot-candles. 


122 


TESTING   OF   ELECTRICAL   MACHINERY 


The  current  through  the  working  standard  is  next  adjusted  until 
equality  of  illumination  is  secured,  and  this  value  is  used  in  all 
subsequent  determinations.  The  arrangement  of  the  controller 
permits  the  reading  of  current  through  either  the  primary 
standard  or  working  standard. 


Cross  Section  of  Macbeth  Illuminometer. 
2 


Connections  of  Controller. 


Cross  Section  of  Reference  Standard. 


FIG.  55 

After  calibration,  the  primary  standard  should  be  discon- 
nected from  the  controller,  and  replaced  in  the  instrument  case. 
The  test  plate  is  then  placed  at  any  point  where  the  illumination 
is  desired  and,  after  adjusting  the  lamp  current  to  the  calibration 
value,  the  illuminometer  is  sighted  on  the  test  plate,  taking 
care  that  no  shadows  are  thrown  on  the  test  plates.  The  illu- 
minometer should  be  held  about  5  feet  from  the  test  plate  in  such  a 
position  that  the  angle  between  the  axis  of  the  telescope  and 
sighting  aperture  and  the  normal  to  the  test  plate  does  not  exceed 
about  40°.  The  position  of  the  rachet  bar  (and  thus  the  posi- 
tion of  the  standard  lamp)  is  varied  until  the  two  illuminations 


ILLUMINATION   LAWS   AND   MEASUREMENTS 


123 


of  the  screen  equalize.  The  reading  on  the  rachet  bar  scale 
then  indicates  directly  the  illumination  in  foot  candles.  The 
action  of  the  illuminometer  itself  is  simUar  to  that  of  the  ordinary 
photometer  bench,  with  structural  modifications  required  by 
the  service  for  which  it  is  intended. 

(b)  G.  E.  Foot-candle  Meter. — This  instrument,  illustrated  in 
Fig.  56,  is  somewhat  more  simple  and  compact  in  its  construc- 
tion than  the  Macbeth  type  previously  described.  It  is  based 
on  the  fact  that  if  a  screen  of  opaque  paper,  with  a  translucent 
grease  spot  at  its  center,  be  equally  illumined  on  both  sides, 
the  grease  spot  will  disappear  and  so  indicate  equalization.  If 
the  light  on  one  side  is  increased  or  decreased,  the  grease  spot 


Voltmeter  Rheostat 


^— 
II     nil 
V^— 


Translucent 
P"Per 

Clear  Glass 


(a)  Front  View  Showing 
Internal  Arrangement. 

FIG.  56 


(b)  Cross  Section 

through 
Light  Box.. 


will  appear  darker  or  lighter,  respectively,  than  the  surrounding 
paper. 

The  row  of  greased  paper  spots  (Fig.  56)  are  illumined  from 
within  by  the  working  standard,  while  the  illumination  on  the 
front  surface  is  unknown,  and  the  value  is  to  be  determined. 
The  working  standard  lamp,  being  placed  at  one  end  of  the  row, 
gives  unequal  illumination  to  the  different  spots,  those  nearest 
the  lamp  receiving  the  greatest  illumination. 

Underneath  the  row  of  spots  is  a  foot-candle  scale.  Thus, 
the  reading  underneath  the  spot  which  disappears  gives  at  once 
the  illumination  on  the  front  surface  of  the  opaque  sheet.  A 
rheostat  and  voltmeter  are  used  to  adjust  the  current  through 
the  lamp  to  its  calibration  value. 


124  TESTING   OF   ELECTRICAL  MACHINERY 

If  the  illumination  (in  lumens  per  square  foot)  be  determined 
for  a  number  of  squares  marked  off  in  a  room,  the  summation  of 
the  products  of  each  area  multiplied  by  its  illumination  gives  the 
total  illumination  effective  at  the  reference  plane,  or 

Lumens  effective  =  2  Illumination  of  Square  X  Area  oi  square. 

Knowing  the  total  lumens  emitted  by  the  lighting  installa- 
tion the  "  utilization  constant "  may  be  at  once  determined, 
where 

•TT4.T     +•      r«  Total  lumens  effective  at  reference  plane 

Utilization  Constant  =  — £ —  . 

Total  lumens  emitted 

The  utilization  factor  is  a  measure  of  the  overall  efficiency  of 
the  installation  and  varies  from  65  per  cent  to  10  per  cent,  depend- 
ing on  the  type  of  reflectors  used  (if  any)  and  the  color  of  the 
ceiling  and  walls. 

The  tests  to  be  carried  out  are  as  follows : 

(a)  Determine   the   M.H.C.P.    and   efficiency    /M-H-C-PA 

\     watts     / 

of  a  carbon,  Mazda  B  (vacuum),  and  Mazda  C  (gas-filled) 
incandescent  lamp,  using  the  photometer  bench  and  connections 
indicated  in  Fig.  57.  A  calibrated  standard  lamp  will  be  avail- 
able for  these  tests. 

(b)  Using  the  Macbeth  Illuminometer  and  General  Electric 
Foot-candle   meter,    determine    the   utilization   constant   of   a 
typical  lighting  installation. 

(c)  Check  the  calibration  of  the  working  standard  of  the 
Macbeth  Illuminometer  against  the  standard  lamp  used  in  (a). 
This  determination  should  check,  within  3  per  cent,  the  value 
of   3   foot-candles   previously   obtained    against    the    primary 
standard. 

(d)  Using  the  Macbeth  Illuminometer  and  the  integrating 
sphere,  determine  the  M.S.C.P.  of  the  four  lamps  tested  in  (a). 
Calculate  the  reduction  factor  for  each  lamp. 

Conclusions,     i.  In  a  photometer,    arranged  as    in  Fig.  57 
the  standard  lamp  is  134  cms.  from  the  screen  and  the  test 


ILLUMINATION   LAWS   AND   MEASUREMENTS 


125 


lamp   is    158   cms.    from   the   screen,  when   balanced.     If   the 
standard    lamp  is   20   cp.,   and    the    reduction    factor    of  the 


Test  Lamp 
(Rotating) 


Screen  Screen 

^  Photometer  Head 


H 


Standard 
Lamp 


Photometer  Bar 


120V- B.C. 


«» 


-<•> 


T 
£' 


FIG.  57 

test    lamp  is  78    per  cent,  what  is  the  M.S.C.P.   of  the  test 
lamp? 

2.  In  a  2oX30-ft.  room,  there  are  four  20  c.-p.  (M.S.C.P.) 
incandescent    lamps    sym- 
metrically arranged  as  in- 
dicated   in  Fig.  58.    What 

is  the  illumination  at  cen- 
ters of  squares  marked  a 
and  b?  What  is  the  utili- 
zation constant  of  the  in- 
stallation? Lamps  are  each 
8  ft.  above  the  floor.  As- 

n      .-  r  FlG-  58 

sume  no  reflection  from 
side  walls  or  ceiling. 

3.  Compare   the  illuminometer  and  the  foot-candle  meter 
with  reference  to 

1 .  Cost  of  construction. 

2.  Accuracy. 

3.  Simplicity  and  ease  of  operation. 

4.  Liability  to  trouble. 

5.  Calibration. 


LIST  OF  A.  C.  EXPERIMENTS 


1.  WAVE  SHAPE,  POWER  AND  POWER  FACTOR,  EFFECTIVE  VALUES. 

2.  PROPERTIES  OF  THE  ALTERNATING  CURRENT  CIRCUIT. 

3.  THE  ALTERNATOR;  ITS  CHARACTERISTICS  ON  NON-INDUCTIVE  AND 

INDUCTIVE  LOAD;  PREDICTION  OF  EXTERNAL  CHARACTERISTIC. 

4.  THE  TRANSFORMER;   OPERATION  AND  CHARACTERISTIC  CURVES; 

MEASUREMENT  OF  LOSSES  AND  PREDICTION  OF  EFFICIENCY. 

5.  THE  INDUCTION  MOTOR;  ITS  OPERATING  CHARACTERISTICS  WITH 

AND  WITHOUT  ADDED  ROTOR  RESISTANCES. 

6.  THE  SYNCHRONOUS  MOTOR;   SYNCHRONIZING  AND  PHASE  CHAR- 

ACTERISTICS. 

7.  THE  SYNCHRONOUS  CONVERTER;    EFFECT  OF  SPEED  AND  VOLT- 

AGE UPON  RATIO;    OPERATING  CHARACTERISTICS. 

8.  PARALLEL  OPERATION  OF  ALTERNATORS;  DISTRIBUTION  OF  LOAD ; 

CIRCULATING  CURRENT,  ETC. 

9.  THREE-PHASE  CIRCUITS;    CURRENT  AND  VOLTAGE  RELATIONS; 

MEASUREMENT  OF  POWER. 

10.  SINGLE-PHASE  MOTORS;  INDUCTION,  REPULSION-INDUCTION,  AND 

SERIES. 

11.  THE  ALTERNATING-CURRENT  WATT-HOUR  METER. 

127 


TESTING  OF  ELECTRICAL  MACHINERY 

ALTERNATING  CURRENT  TESTS 


EXPERIMENT   I 

Wave  Shape,  Effective  Values,   Power  and   Power   Factor. 

An  alternating  current  is  one  which  periodically  changes  its 
direction  of  flow.  The  frequency  (number  of  cycles  or  periods 
per  second)  depends  upon  the  class  of  service  for  which  the 
alternating  current  power  is  to  be  used.  In  alternating  current 
railway  motors  a  frequency  of  15  or  25  is  employed;  for  illumi- 
nation, arc  and  incandescent  lamps,  a  frequency  of  60  is  stand- 
ard; telephone  currents  are  made  up  of  many  superimposed 
frequencies,  ranging  from  perhaps  100  to  1500;  for  wireless 
telegraphy  and  telephony  the  frequency  may  be  between  20,000 
and  several  hundred  thousand;  two  small  metallic  spheres 
charged  with  electricity  of  opposite  kinds  will,  if  brought  close 
enough  together,  exchange  and  neutralize  charges  ;  the  discharge 
current  is  oscillatory  in  character  and  may  be  of  a  frequency  of 
several  billions  of  cycles  per  second. 

Generally  an  alternating  current  is  sinusoidal  in  form. 

If  i  =  instantaneous  value  of  current; 

/max  =  maximum  value  of  current; 

p  =  2nX  frequency  of  current; 
we  have 

*  =  /max  COS   pt. 

An   alternating   current   generator   is   generally   designed    so 
that  its  wave  form  may  be  expressed  by  the  formula 


129 


130  TESTING   OF   ELECTRICAL   MACHINERY 

Whether  or  not  a  generator  gives  such  a  wave  form  depends 
upon  the  shape  of  the  air  gap  between  armature  and  pole  face 
and  also  upon  the  distribution  of  armature  winding. 

Whenever  a  sine  wave  of  E.M.F.  is  applied  to  a  circuit,  the 
current  which  is  caused  to  flow  is  also  a  sine  wave  (neglecting 
distorting  effects  of  hysteresis,  variation  in  permeability,  dielec- 
tric loss  in  condensers,  etc.).  If  the  circuit  is  non-inductive  the 
E.M.F.  and  current  waves  are  in  phase,  i.e.,  the  maximum  and 
minimum  values  of  the  two  waves  occur  simultaneously.  If, 
however,  the  circuit  offers  inductance  or  capacity  reaction,  or 
both,  the  current  may  either  lag  or  lead  the  E.M.F.  in  phase, 
depending  upon  which  reaction  predominates.  These  reactions 
are  more  fully  analyzed  in  the  discussion  of  Ex.  2. 

The  question  arises  how  much  power  is  used  in  a  circuit 
in  which  an  alternating  current  is  flowing?  The  reactions 
which  are  offered  to  the  flow  of  the  current  are  of  two  general 
types,  conservative  or  non-dissipative,  and  non-conservative 
or  dissipative. 

The  product  of  the  current  into  the  dissipative  reaction  gives 
the  rate  at  which  power  is  being  used  in  the  circuit;  the  con- 
servative reaction  causes  no  energy  loss. 

If  a  weight  suspended  from  a  spiral  spring  is  forced  to  vibrate 
at  its  natural  frequency,  with  constant  amplitude,  the  only  force 
required  will  be  that  necessary  to  overcome  the  frictional  resist- 
ance of  the  moving  system.  The  other  reactions  in  the  system 
as  shown  to  exist  by  the  stretching  of  the  spring  (to  a  much 
greater  degree  than  the  impressed  force  could  directly  bring 
about)  are  the  conservative  reactions  of  the  system;  they  are  the 
forces  represented  by  the  stretching  of  the  spring  and  the  change 
in  momentum  of  the  moving  weight.  The  power  which  is 
expended  in  maintaining  constant  amplitude  of  oscillation  of  the 
weight  is  equal  to  the  product  of  the  velocity  of  the  weight  into 
the  frictional  resistance  of  the  system. 

The  rate  at  which  energy  is  dissipated  from  an  electrical 
circuit  is  equal,  in  the  same  way,  to  the  product  of  the  current 
into  the  total  dissipative  reaction  in  the  system. 


WAVE  SHAPE,  EFFECTIVE  VALUES  Id 

If  i  =  instantaneous  value  of  current, 

R  =  effective  resistance  (see  Ex.  2)  of  circuit,  then  the  dissi- 
pative  reaction  at  any  instant  =  iR}  and  the  rate  at 
which  energy  is  being  used  is  equal  to  iXiR  =  i2R. 

As  this  rate  varies  throughout  the  cycle  it  is  necessary  to  get 
the  average  rate  at  which  the  power  is  used. 

i   f* 

Average  power  =  -  I    i2R  dt. 
xjo 

Now  if  i  =  Imaxcospt, 

j'2  ~D    f*  i:  TO 

average  power  =   '^^\    cos2  pt  d(pt)  =  -^R. 

Now  if  a  direct  current  is  passed  through  the  same  resistance 
the  rate  at  which  energy  is  dissipated  is  given  by  the  formula  : 

Power  =  I2R. 

If  the  alternating  current  is  varied  in  amplitude  until  it  is 
supplying  the  same  amount  of  power  as  is  the  direct  current, 
we  have 


or 

j  _  -*max 

~ 


and  this  is  called  the  effective  value  of  the  alternating  current. 
It  equals  0.707  of  the  maximum  value.  By  the  same  line  of  reason- 
ing the  effective  value  of  voltage  of  a  sinusoidal  alternating  E.M.F. 
is  equal  to  0.707  the  maximum  value.  A.C.  ammeters  and  volt- 
meters are  always  calibrated  to  read  effective  values. 

It  is  to  be  noticed  that  if  the  wave  of  current  or  E.M.F.  is 
not  sinusoidal,  the  previous  integration  is  not  so  simple,  but 
involves  a  number  of  terms  of  a  Fourier  series.  The  effective 
value  of  such  wave  is  not  equal  to  0.707  the  maximum  value,  but 
varies  with  the  wave  shape.  For  this  reason  all  standard  tests 
on  alternating  current  apparatus  must  be  carried  out  with  sin- 
usoidal wave  forms;  otherwise  inaccuracies  are  likelv  to  occur. 


132  TESTING    OF  ELECTRICAL  MACHINERY 

It  has  been  mentioned  that  if  conservative  reactions  occur 
in  a  circuit  the  current  and  voltage  will  not  generally  be  in  the 
same  phase.  The  dissipative  reaction  is  equal  to  that  com- 
ponent of  the  impressed  force  which  is  in  phase  with  the  current. 

If  0  =  angular   difference    in    phase    of    current    and 

impressed  E.M.F.; 

£max  cos  $  =  maximum  value  of  dissipative  reaction  ; 
—  ^^42^  =  effective  value  of   dissipative  reaction. 

\/2 

If  J  =  effective  value  of  current; 


,.     ,  maxOS   </> 

Power  supplied  =  -  -=.  —  - 
\/2; 

=  /Ecos0; 
•p 

where  E  =  —  ^=  effective  value  of  impressed  E.M.F. 

2 


It  is  therefore  evident  that  the  product  of  E  and/  as  obtained 
from  A.C.  meters  does  not  generally  represent  the  power  used  in 
a  circuit.  It  is  necessary  to  know  cos  <p  to  obtain  true  watts. 
The  product  El  is  sometimes  called  "  apparent  watts  "  in  dis- 
tinction to  El  cos  <f>,  the  true  power  of  the  circuit  in  watts. 

This  quantity,  cos  <f>,  is  extremely  important  in  all  alternating- 
current  work.  It  is  called  the  power  factor  of  the  circuit  and  is. 
that  quantity  by  which  the  volt-amperes  of  the  circuit  must 
be  multiplied  to  give  the  power,  in  watts,  in  the  circuit. 

If  we  have  some  method  of  actually  plotting  the  waves  of 
current  and  E.M.F.  in  a  circuit,  in  proper  magnitude  and  phase, 
the  power  El  cos  <p,  could  be  immediately  obtained.  E  and  / 
are  calculated  by  multiplying  the  maximum  values  of  the  respect- 
ive waves  by  0.707;  <f)  is  measured  as  the  angular  distance  between 
two  corresponding  points,  say  the  zero  points,  of  the  two  waves, 
remembering  that  a  complete  cycle  represents  360°. 

Generally  it  would  be  inconvenient  and  slow  to  use  such  a 
method;  so  a  wattmeter  is  used.  This  instrument  has  two  coils, 
one  of  low  resistance  and  comparatively  large  cross-section  and 


WAVE  SHAPE,  EFFECTIVE  VALUES  133 

the  other  of  very  fine  wire,  of  high  resistance;  it  is  connected 
to  the  circuit,  in  which  the  power  is  to  be  measured,  as  though 
an  ammeter  and  voltmeter  were  in  the  same  case;  the  two  coils 
are  so  placed  in  relation  to  one  another  and  the  scale  is  so  cali- 
brated that  the  reading  of  the  instrument  is  actually  equal  to 
El  cos  <j>.  The  current  coil  of  the  instrument  is  placed  in  series 
with  the  circuit  the  power  of  which  it  is  desired  to  measure,  and 
the  potential  coil  is  connected  in  parallel  with  the  circuit.  Unlike 
the  A.C.  ammeter  and  voltmeter,  which  cannot  be  so  connected 
to  an  A.C.  circuit  that  the  needle  is  deflected  backward,  the  watt- 
meter may  deflect  backward  with  a  wrong  connection.  The 
connection  of  either  the  current  or  potential  coil  must  be  reversed 
in  such  case. 

It  is  the  object  of  this  experiment  to  plot  curves  of  E.M.F. 
and  current  in  an  inductive  circuit,  in  their  proper  phase  and 
magnitude,  and  to  measure  with  A.C.  meters  the  E.M.F.,  current, 
and  power  of  the  circuit  in  order  to  verify  the  previously  stated 
facts  regarding  effective  values,  power,  cos  <f>,  etc. 


Receiver 


FIG.  i 

The  method  to  be  employed  is  commonly  called  that  of 
"  instantaneous  contact."  It  may  be  easily  explained  by  refer- 
ence to  a  direct  current  circuit.  In  Fig.  i  are  shown  two  cells 
and  a  telephone  receiver  (any  other  sensitive  current  indicator 
would  work  as  well  as  a  telephone  receiver).  When  switch  S 
is  closed  no  current  will  flow  provided  that  the  two  E.M.F. 's 
in  the  circuit  are  opposite  and  just  equal.  If,  however,  the  two 
E.M.F. 's  are  not  equal  and  opposed,  current  will  flow  every 
time  the  switch  is  closed.  If  the  switch  is  closed  and  opened  at 
regular,  short,  intervals  of  time  a  more  or  less  musical  note  will 


134 


TESTING  OF  ELECTRICAL  MACHINERY 


be  heard  in  the  telephone  receiver.  Suppose  now  this  principle 
is  applied  to  an  alternator  as  shown  in  Fig.  2.  On  the  same 
shaft  with  the  alternator  armature  C,  is  mounted  a  disc  of  some 
insulating  material.  A  metal  strip  H,  inserted  in  the  disc, 
reaches  to  the  periphery  and  is  connected  at  its  inner  end  to  a 
small  conducting  drum  and  so  to  brush  F.  A  brush  E  rests 
on  the  periphery  of  the  disc  and  is  insulated  from  the  alternator 
frame.  It  will  be  noticed  that  this  combination  of  brushes, 
metal  strip,  and  insulating  disc  is  nothing  but  a  rotating  switch, 
the  two  brushes  F  and  E  being  connected  together  once  for  each 
revolution  of  the  armature;  moreover,  whenever  this  contact  is 
made,  the  armature  occupies  the  same  position  in  the  field. 
Now  the  E.M.F.  wave  generated  by  the  revolving  armature  has 
the  same  value  every  time  the  armature  occupies  the  same  posi- 


tion  in  its  field;  in  other  words,  every  time  the  brushes  F  and  E 
are  connected  together  the  instantaneous  value  of  the  E.M.F. 
is  the  same. 

A  variable  resistance  A,  connected  to  a  supply  of  constant  volt- 
age, serves  as  a  source  of  variable  potential  difference;  by  the  sliding 
contact  B,  the  P.D.  between  G  and  B  may  be  made  anything 
desired.  The  local  circuit,  BAGEFC,  is  exactly  similar  in  its, 
make  up  to  that  of  Fig.  i,  except  that  the  two  E.M.F. 's  are 
varying.  The  rotating  switch  closes  the  local  circuit  at  the  same 
point  on  successive  E.M.F.  waves  of  the  alternator,  shown  at 
a,  a',  a",  in  Fig.  3.  If  the  slide  B  is  moved  on  the  potentiometer 
A,  until  the  P.D.  between  G  and  B  is  just  equal  to  the  voltage 


WAVE  SHAPE,  EFFECTIVE  VALUES 


135 


a,  then  when  the  rotating  switch  closes,  no  current  will  flow  through 
the  telephone  receiver  and  hence  no  sound  will  be  heard.  A 
D.C.  voltmeter,  connected  to  B  and  G,  will  then  give  the  value 
of  the  D.C.  voltage  necessary  to  balance  the  instantaneous  value 


FIG.  3 


of  the  A.C.  voltage;  i.e.,  it  will  give  the  instantaneous  value  of 
the  A.C.  voltage.  If  now  the  brush  E  is  moved  around  on  the 
periphery  the  local  circuit  will  be  closed  when  the  A.C.  voltage 
is  perhaps  bb'  b"  (Fig.  3).  The  sliding  contact  B  (Fig.  2)  must 


FIG.  4 


then  be  moved  to  reduce  the  telephone  sound  again  to  zero,  when 
the  D.C.  voltmeter  will  give  the  value  of  this  voltage  b  (Fig.  3). 
So  by  moving  E  (Fig.  2)  through  360  electrical  degrees,  taking 
readings  at  every  few  degrees  (say  15°)  the  entire  wave  of  E.M.F. 
may  be  plotted,  point  by  point.  The  actual  connections  to  be 
made  for  this  test  are  shown  in  Fig.  4.  The  plug  switch  makes 


136 


TESTING    OF    ELECTRICAL    MACHINERY 


it  possible  to  connect  the  telephone  circuit  to  any  one  of  the  three 
E.M.F.  waves  to  be  measured. 

An  inductive  coil  A  and  resistance  C  are  connected  in  series 
and  through  the  current  coil  of  wattmeter  W,  through  ammeter 
A  to  the  A.C.  generator.  Taps  are  taken  off  the  circuit  and 
connected  to  receptacles  so  that  the  jack  can  connect  the  telephone 
circuit  either  to  the  terminals  of  A,  C,  or  the  entire  circuit.  For 
each  setting  of  the  brush  B,  a  balance  is  to  be  obtained  with  the 
plug  inserted  in  each  of  the  receptacles  in  turn;  the  brush  is  to  be 
moved  in  steps  of  about  15°  (electrical)  until  the  three  waves  are 
obtained  for  a  complete  cycle. 


Degrees  'Q 


FIG.  5 


The  wave  obtained  from  receptacle  No.  i  gives  the  phase 
of  the  current,  as  the  current  and  E.M.F.  are  in  phase  on  a  non- 
inductive  circuit.  The  wave  from  receptacle  No.  3  gives  the 
impressed  voltage,  and  so  is  the  same  as  the  terminal  voltage  of 
the  alternator  and  shows  how  nearly  the  alternator  generates  a 
sine  E.M.F. 

The  inductance  used  should  have  an  air  core,  otherwise  the 
current  will  be  distorted  and  the  value  of  <f>  obtained  from  the 
curve  sheet  will  be  difficult  to  interpret.  After  the  circuit  has 
been  properly  adjusted  to  give  readable  deflections  on  the  meters 
used,  take  a  reading  of  volts,  amperes  and  watts  for  each  part  of 


WAVE  SHAPE,  EFFECTIVE  VALUES 


13? 


the  circuit,  and  for  the  whole  circuit.  To  read  watts  used  in 
any  part  of  the  circuit,  connect  the  potential  leads  of  the  wattmeter 
across  that  part  of  the  circuit. 

Maintain  the  impressed  voltage  and  frequency  constant  and 
determine  the  three  curves  of  voltage. 

The  impressed  voltage  must  not  be  greater  than  about  65 
per  cent  of  the  D.C.  voltage  used  on  the  potentiometer  resistance. 

The  curves  obtained  should  look  about  as  those  given  in  Fig. 
5.  C  is  the  drop  across  the  resistance,  hence  gives  the  phase 
and  form  of  the  current;  B  is  the  drop  across  the  inductance, 
and  E  is  the  impressed  E.M.F.  The  angle  of  lag  for  the  entire 
circuit  is  equal  to  b,  measured  in  degrees;  the  phase  displacement 
of  current  and  E.M.F.  in  the  inductance  coil  is  given  by  a. 

A  vector  diagram  showing  the  same  relations  as  those  given 
by  the  curves  of  Fig.  5  is  shown  in  Fig.  6;  the  direction  of 


Direction  of 
Vector  Rotation 


I 

I      Phase  of  Current 


C 


FIG.  6. 


positive   vector  rotation  is  taken  counter-clockwise  as  is   the 
practice  in  all  alternating-current  diagrams. 

The  phase  of  the  current  is  taken  as  reference  vector,  and  in 
phase  with  this  current  is  the  voltage  OC  which  is  the  component 
of  the  impressed  voltage  required  to  overcome  the  reaction  of  the 
resistance  C  of  Fig.  4.  At  OB  is  shown  the  component  of 
impressed  voltage  required  to  overcome  the  reactions  in  the 
inductance  shown  at  A  in  Fig.  4;  this  voltage  will  be  nearly 
90°  out  of  phase  with  the  current  if  the  resistance  of  the  induct- 


138  TESTING   OF   ELECTRICAL   MACHINERY 

ance  coil  is  small  compared  to  its  reactance.  The  impressed 
voltage  OE  must  be  of  such  magnitude  and  phase  as  to  supply 
the  two  required  drops  OB  and  OC. 

The  angle  <£  of  Fig.  6  is  equal  to  the  angle  indicated  at  b  in 
Fig.  5;  it  may  have  nearly  any  value  between  zero  and  90° 
depending  upon  the  relative  values  of  reactance  and  resistance  in 
the  circuit. 

Check  the  values  of  cos  0,  obtained  from  the  plotted  curves, 
with  the  values  computed  from  readings  of  wattmeter,  ammeter 
and  voltmeter.  With  the  same  base  and  maximum  value  as 
curve  E,  construct  a  sine  curve  to  see  how  closely  the  generated 
E.M.F.  approximates  this  shape.  Compare  the  voltmeter 
readings  of  the  three  parts  of  the  circuit  with  the  effective  values 
as  computed  from  the  curve  sheet  (o. 707 X maximum  value). 

QUESTIONS 

What  difficulty  would  be  encountered  in  this  test  if  the 
voltage  across  the  circuit  as  read  on  the  alternating  current 
voltmeter  was  85,  while  the  voltage  across  the  potentiometer 
was  105? 

If  the  readings  on  a  certain  circuit  were  60  volts,  2.5  amperes, 
85  watts,  what  should  be  the  distance  b  of  Fig.  5? 

What  relation  exists  between  the  instantaneous  values  of 
voltage  across  each  part  of  the  circuit  and  across  the  whole 
circuit? 

Why  should  the  strap  H  and  the  brush  E  (of  Fig.  2  )  be  as 
narrow  as  possible? 


EXPERIMENT  II 

Properties  of  the  Alternating  Current  Circuit.  The  object 
of  this  test  is  to  investigate  the  relations  existing  between  the 
phase  and  magnitude  of  the  current  and  impressed  E.M.F.  of 
a  circuit  containing  inductance,  capacity  and  resistance  in  series. 
Also  the  effect  of  frequency  is  to  be  noted  and  the  difference 
between  ohmic  and  "  effective  "  resistance  is  to  be  measured. 

When  an  E.M.F.,  either  A.C.  or  D.C.,  is  impressed  upon  any 
circuit,  a  current  is  caused  to  flow,  the  value  of  which  depends 
upon  the  reactions  which  the  circuit  offers.  When  the  char- 
acteristics of  the  circuit  are  known,  the  value  of  the  current  may 
always  be  found  by  setting  up  the  differential  equation  of  reac- 
tions existing  in  the  circuit,  and  putting  their  sum  equal  to  the 
impressed  E.M.F.  In  general  the  reactions  are  proportional 
to  the  current  or  some  function  of  the  current,  so  that  the  dif- 
ferential equation  involves  the  current,  constants  of  the  circuit, 
and  impressed  force;  its  solution  expresses  the  current  in  terms 
of  the  impressed  force  and  the  constants  of  the  circuit. 

In  the  direct  current  circuit  the  solution  is  extremely  simple, 
because  the  current  itself  is  generally  a  constant  quantity,  but 
in  the  case  of  alternating  currents  the  reactions  are  more  complex 
and  so  the  solution  is  more  difficult.  The  solution  will  not  be 
attempted  here,  but  the  results  of  the  solution  will  be  used  and 
discussed. 

The  three  reactions  which  occur  in  the  circuit  to  be  tested 
are  resistance  reaction,  inductance  reaction  and  capacity  reac- 
tion; they  will  each  be  discussed  separately. 

In  the  ordinary  D.C.  circuit,  as  e.g.,  an  incandescent  lamp 

139 


140  TESTING   OF   ELECTRICAL   MACHINERY 

connected  to  a  supply  line,   Ohm's  law  expresses  the  relation 
existing  between  current  and  E.M.F., 

E  =  IR 
where 

E= potential  difference  at  terminals  of  lamp; 
/= current  flowing  through  lamp; 
R  =  resistance  of  lamp. 

In  case  an  A.C.  E.M.F.  is  applied  to  this  lamp,  exactly  the 
same  law  holds;  E  and  /  will  be  the  effective  values  of  the  voltage 
and  current  while  R  will  have  exactly  the  same  value  as  it  had 


FIG.  7 


for  the  direct  current.  This  is  the  case  of  an  A.C.  circuit  having 
only  resistance  reaction;  moreover,  the  effective  resistance  is 
the  same  as  the  ohmic  resistance.  (This  will  always  be  true, 
for  ordinary  frequencies,  unless  the  circuit  embraces  a  magnetic 
path  made  up  of  iron.  The  difference  will  be  discussed  later.) 

The  above  law  gives  the  magnitude  of  the  alternating  current 
through  the  lamp,  and  its  phase  will  be  the  same  as  that  of  the 
E.M.F.  As  R  is  a  constant  the  current  will  at  every  instant  be 
directly  proportional  to  the  E.M.F.  If  the  voltage  is  a  sine  curve, 
the  current  will  also  be  a  sine  curve  (in  general  of  different  ampli- 
tude) in  exactly  the  same  phase  as  the  voltage.  The  curve 
representation  of  the  two  variables  is  given  in  Fig.  7. 


PROPERTIES  OF  THE  ALTERNATING  CURRENT  CIRCUIT        141 

The  next  reaction  to  be  considered  is  that  offered  by  a  coil 
of  wire  having  a  considerable  number  of  turns.  (A  coil  of  few 
turns  shows  the  same  effect  as  one  having  many  turns,  but 
not  to  an  extent  sufficient  to  measure  with  ordinary  laboratory 
instruments.)  A  current  flowing  through  such  a  coil  produces 
a  magnetic  field  the  strength  of  which  is  at  any  instant  directly 
proportional  to  the  value  of  the  current.  It  is  a  property  of  such 
a  magnetic  field  that  whenever  its  strength  is  changed  an  electro- 
motive force  is  set  up  in  the  electric  circuit  which  embraces  the 
magnetic  field.  The  direction  of  this  E.M.F.  of  self  induction 


N        -x /         Resistance 

'^C      \         /•*<  V  reaction 


FIG. 


depends  upon  whether  the  field  is  increasing  or  decreasing,  and 
its  magnitude  depends  upon  the  rate  of  change  of  the  field  and 
number  of  turns  in  the  coil.  As  the  field  is  always  proportional 
to  the  current,  it  is  evident  that  this  inductance  reaction  is  pro- 
portional to  the  rate  of  change  of  the  current. 

It  is  evident,  therefore,  that  if  an  alternating  current  is  sent 
through  such  a  coil,  there  will  be  two  reacting  forces  set  up  in 
the  circuit,  the  resistance  reaction  (the  coil  will  of  course  have 
resistance)  and  the  inductance  reaction.  If  the  current  is  a 
sine  function  of  the  time  the  rate  of  change  of  the  current,  to 
which  the  inductance  reaction  is  proportional,  will  be  of  similar 
form  but  displaced  90°  from  it.  By  further  analysis  it  may  be 
shown  that  this  inductance  reaction  is  90°  behind  the  phase  of 
the  current.  The  two  reactions  may  be  presented  in  the  form 


142 


TESTING   OF   ELECTRICAL  MACHINERY 


of  curves  as  shown  in  Fig.  8.  The  current  is  assumed  in  magni- 
tude and  phase,  the  resistance  and  inductance  reactions  can  then 
be  plotted,  and  the  impressed  force  is  then  found  as  equal  to  the 
sum  of  the  two  reactions  and,  of  course,  opposite  in  phase.  Rep- 
resented vectorially  it  will  be  seen  that  the  different  forces  of  Fig.  8 
are  properly  given  in  Fig.  9.  The  phase  of  the  current  is  assumed 
as  01.  The  resistance  reaction  is  shown  as  OR  and  the  inductance 
reaction  as  OX.  The  combined  reactions  are  then  given  by 
the  vector  OZ;  the  impressed  force  must  be  equal  and  opposite 


FIG.  9 


to  this,  so  is  properly  shown  as  OE.  The  length  of  the  vector 
OR  is  equal  (in  scale  of  volts)  to  the  current  (in  amperes)  multi- 
plied by  resistance  (in  ohms).  Maximum  values  of  E.M.F. 
and  current  would  naturally  be  used  in  constructing  the  vector 
diagram,  but  the  diagram  hold  s  good  if  effective  values  (maximum 
value  /\/2)  are  used,  as  it  simply  amounts  to  a  change  in  scale. 

The  length  of  the  vector  OX  is  proportional  to  L,  the  coef- 
ficient of  self  induction  of  the  circuit  expressed  in  henrys,  and 
to  the  rate  of  change  of  the  current.  But  if  the  current  is  ex- 
pressed by  2  = /max  sin  27r//  it  is  at  once  evident  that  the  maximum 
value  of  the  inductance  reaction  is  27:/L/max.  But  if  the  other 


PROPERTIES  OF  THE  ALTERNATING  CURRENT  CIRCUIT     143 

reactions  are  expressed  in  effective  values,  the  inductance  reaction 
is  given  by  OX  =  2nfLI,  where  /  is  the  effective  value  of  the  current. 
The  phase  difference  of  the  impressed  force  is  generally  desig- 
nated by  (j>  and  from  the  diagram  it  is  seen  that 


JT/Z 

- 


R 


or     cos  9  = 


The  power  used  up  in  the  circuit  is  equal  to  the  current  X resist- 
ance reaction.  But  resistance  reaction  =  impressed  E.M.F.  Xcos  (/>. 
So  that  power  used  up  (expressed  in  watts)  =IXIR  =  IE  cos  <j>. 

It  was  proved  in  Ex.  i  that  the  wattmeter  reading  does  give 
El  cos  d>. 


Capacity  reaction^ 


urrent 


FIG.  10 


If  now  a  condenser  is  put  in  the  circuit  a  capacity  reaction 
will  result.  The  counter  E.M.F.  of  a  condenser  is  equal  to 
QIC 

where  Q  =  quantity  of  electricity  in  condenser; 
C  =  capacity  of  condenser. 

If  Q  is  expressed  in  coulombs,  and  C  in  farads,  the  counter 
E.M.F.  will  be  given  by  the  formula  in  volts. 

The  quantity  of  electricity  in  a  condenser,  of  positive  polarity, 
e.g.,  will  be  a  maximum  at  the  end  of  a  positive  alternation  of  the 
current.  When  the  current  reverses,  some  of  the  electricity 
begins  to  flow  out  of  the  condenser,  so  that  the  truth  of  the  above 
statement  is  evident.  The  condenser  reaction  will  therefore 


144  TESTING   OF   ELECTRICAL   MACHINERY 

have  a  maximum  positive  value  at  the  end  of  a  positive  loop  of 
current  and  the  reaction  is  of  the  same  form,  with  respect  to  time, 
as  the  current  wave  (if  the  current  is  a  simple  sine  wave).  The 
phase  relations  between  current  and  condenser  reaction  is  shown 
in  Fig.  10  and  vectorially  in  Fig.  u.  In  \his  latter  figure  OI 
represents  the  current  and  OC  gives  the 
condenser  reaction.  The  magnitude  of 
this  reaction  is 


where  Ec  =  capacity  reaction  in  volts ;  /  = 
current  in  amperes;   /=  frequency;  C=          Q 

.        P  ,  FlG.    II 

capacity  in  farads. 

The  three  reactions  which  have  been  discussed  may  now  be 
grouped  for  convenience : 

Resistance  reaction  =IR,  180°  out  of  phase  with  current; 

Inductance  reaction  =  2nfLI,  90°  behind  current, 

Capacity  reaction     = — — ,  90°  ahead  of  current. 

The  quantity  2xfL  is  called  the  reactance  of  an  inductive  cir- 
cuit;   for  a  condensive  circuit  the  quantity  — —   is  called  the 

27T/C 

reactance.     In  case  both  condenser  and  inductance  are  present, 
and  connected  in  series  the  reactance  is  equal  to  \2nfL  - — - — ) 

\  27T/C/' 

The  reactance  is  generally  designated  by  the  letter  X,  so 
for  a  condenser 

X= 


—  77=1) 
27T/C 

and  for  an  inductance,          X=2nfL\ 
and  if  both  are  connected  in  series, 

X=27lfL-      * 

271  fC 

When  an  inductance,  resistance  and  condenser  are  connected 
in  series  the  relation  between  voltage  impressed  and  current  is 


PROPERTIES  OF  THE  ALTERNATING  CURRENT  CIRCUIT        145 

E 


T  _ 


VR2+X2     Z' 

This  quantity  Z  is  called  the  impedance  of  a  circuit  and  is 
expressed  in  ohms,  just  the  same  as  resistance  or  reactance. 

Generally  vector  diagrams  do  not  give  the  reactions  them- 
selves, but  the  components  of  the  impressed  E.M.F.  used  in 
overcoming  these  reactions.  These  components  are  sometimes 
called  the  reactions;  it  must  be  remembered,  however,  that 
really  the  reactions  are  180°  out  of  phase  with  these  components 
of  the  E.M.F.  In  so  far  as  no  ambiguity  will  arise  from  such 
nomenclature  and  as  text-books  on  the  subject  of  alternating 
currents  generally  use  the  terms  inductance  drop,  etc.,  signi- 
fying "  the  component  of  the  impressed  E.M.F.  to  balance  the 
inductance  reaction  "  such  use  of  the  terms  will  be  made  here. 
We  have,  therefore, 

Current  lags  90°  behind  inductance  drop; 
Current  leads  90°  ahead  of  capacity  drop; 
Current  in  phase  with  resistance  drop. 

The  difference  between  ohmic  and  effective  resistance  is  now 
to  be  noted.  If  current  is  sent  through  one  coil  of  a  transformer 
(a  coil  having  a  magnetic  circuit  in  which  iron  is  used)  and  the 
power  used  in  the  coil  is  measured  by  a  wattmeter,  it  will  be  found 
that  the  Wattmeter  indication  is  much  greater  than  Pr  where 

7  =  the  current, 

r  =  resistance  of  coil,  calculated  from  size  and  length  of  w?re, 
or  measured  by  direct  current  test. 

The  continually  reversing  magnetic  field  in  the  iron  uses  up 
energy  as  hysteresis  and  eddy  current  loss.  This  loss  must  be 
furnished  by  the  line  supplying  the  current,  and  the  wattmeter 
measures  this  loss  and  so  gives  the  same  reading  as  though  the 
ohmic  resistance  of  the  coil  was  much  greater  than  it  actually  is. 


146 


TESTING   OF   ELECTRICAL   MACHINERY 


If  /= current; 

f  =  ohmic  resistance; 

W = watts  consumed,  shown  by  wattmeter; 

R  =  effective  resistance; 
Then  W  =  PR-, 

Since  copper  loss  =  /2r.  we  may  write 


where  x  represents  the  apparent  increase  in  resistance  of  the 
circuit  produced  by  hysteresis  and  eddy  currents  in  the  iron. 
The  value  of  x  depends  upon  the  quantity  and  quality  of  the 
iron  in  the  magnetic  circuit,  the  flux  density  in  the  iron  and  the 
frequency  of  the  current. 


FIG.  12 


PROPERTIES  OF  THE  ALTERNATING  CURRENT  CIRCUIT        147 


If  a  condenser,  inductance  and  non-inductive  resistance 
are  connected  in  series,  all  of  the  previously  discussed  reactions 
will  occur  in  the  circuit.  The  different  reacting  forces  may  be 
measured  and  their  relative  phases  determined.  Added  vector- 
ially  these  component  E.M.F.'s  should  give  the  impressed  E.M.F. 
both  in  phase  and  magnitude.  As  was  shown  in  Ex.  i,  the  watt- 
meter reading  of  any  part  of  the  circuit  gives  the  product  of  the 
resistance  (effective)  reaction  and  the  current.  By  dividing  the 
watts  by  current  in  the  circuit  the  resistance  reaction  is  there- 
fore found.  The  proper  phase  for  the  voltage  drop  in  the  circuit, 
(or  part  of  circuit)  referred  to  the  current,  is  thus  obtained. 
In  Fig.  12,  is  shown  the  vector  diagram  for  the  circuit  as  given 


Inductance 


A.C. 
Supply 


Resistance 


|  I  [  Condenser 


FIG.  13 

in  Fig.  13.  The  phase  of  the  current  is  assumed  as  OI.  The 
non-inductive  resistance  drop,  OR,  is  plotted  in  phase  with  01 
and  equal  in  magnitude  to  the  voltage  across  the  non-inductive 
resistance  as  indicated  by  the  voltmeter.  The  resistance  reaction 
of  the  inductance  coil  is  obtained  by  dividing  the  watts  used  in 
the  inductance  coil  by  the  current.  This  voltage  is  plotted  in 
Fig.  12  as  ORL.  With  a  radius  equal  to  the  voltage  drop  across 
the  inductance  coil  (measured  by  voltmeter)  an  arc  is  drawn 
about  0  as  center  and  the  inductance  drop  is  then  plotted  in 
such  a  phase  as  will  give  the  requisite  component  ORL  in  phase 
with  the  current.  It  is  shown  as  OL,  and  the  resultant  of  OR  and 


148  TESTING   OF   ELECTRICAL   MACHINERY 

OL  is  shown  as  OA.  The  capacity  drop  is  plotted  as  OC.  (It 
is  supposed  that  some  power  is  used  in  th'e  condenser,  giving  a 
voltage  component  in  phase  with  the  current.  This  component 
is  obtained  in  the  same  manner  as  ORL  was  obtained.)  The 
resultant  of  the  three  voltages  OR,  OL  and  OC,  is  shown  as 
OE.  This  vector,  OE}  should  agree  both  in  magnitude  and 
phase  with  the  impressed  voltage  as  measured  and  calculated 
from  readings  of  voltmeter,  wattmeter  and  ammeter. 

It  is  to  be  noticed  that  the  angle  <j>  may  not  check  very  closely 
with  the  value  as  obtained  by  the  formula, 

watts  in  total  circv't 

cosc6  =  —  — ; —  . 

current  X impressed  voltage 

This  will  be  especially  true  if  <j>  is  small.  The  value  determined 
experimentally  is  cos  <f>  and  not  <j>  itself.  When  <j>  is  small  a  large 
change  in  <f>  is  accompanied  by  only  a  small  change  in  cos  <f>. 

With  connections  •made  as  in  Fig.  13,  using  an  inductance 
coil  with  iron  core  and  a  frequency  as  low  as  obtainable  with  the 
generator  used,  adjust  the  different  parts  of  the  circuit  until 
the  drop  in  the  condenser  is  about  50  per  cent  greater  than  the 
resistance  drop  and  the  drop  across  the  coil  is  about  50  per  cent 
smaller  than  the  resistance  drop.  Read  current,  volts  and 
watts  for  each  part  and  for  the  whole  circuit  keeping  the  im- 
pressed voltage  constant  while  getting  the  set  of  readings. 
Then  increase  the  frequency  to  as  high  a  value  as  is  obtainable 
(say  twice  as  much  as  that  used  in  the  first  test),  leaving  the 
circuit  exactly  as  it  was  for  the  low  frequency.  Now  vary  the 
impressed  voltage  until  the  current  flowing  in  the  circuit  is 
just  the  same  as  for  the  low-frequency  run.  Read  current, 
volts  impressed,  and  drop  across  each  part,  and  watts  used  in 
whole  circuit  and  in  each  part. 

Take  two  more  sets  of  readings  after  having  adjusted  L, 
R,  and  C  to  different  values.  Keep  the  current  for  these  two  runs 
the  same,  not  necessarily  the  same  as  for  the  two  former  runs. 

By  direct  current  "drop  of  potential"  method  measure  the 
ohmic  resistance  of  the  inductance  coil,  of  the  condenser,  and  of 


PROPERTIES  OF  THE  ALTERNATING  CURRENT  CIRCUIT      149 

the  non-inductive  resistance.  If  possible  make  these  measure- 
ments of  resistance  with  about  the  same  value  of  current  as  used 
in  the  A.C.  test  to  avoid  errors  due  to  heating. 

It  will  of  course  be  found  that  the  condenser  will  not  take  as 
much  current  on  the  D.C.  test  as  on  the  A.C.  test;  generally  the 
current  in  the  D.C.  test  will  be  so  small  that  an  ordinary  ammeter 
gives  no  discernible  deflection. 

The  resistance  of  the  condenser  is  most  conveniently  measured 
by  use  of  a  voltmeter  of  known  resistance  ;  if  none  is  available  it 
maybe  remembered  that  the  ordinary  portable  direct  current  volt- 
meter, such  as  used  for  laboratory  work,  has  about  80  ohms  resist- 
ance per  volt  of  scale  (thus  a  150  voltmeter  has  about  12,000  ohms 
resistance)  .  Take  the  voltmeter  reading  when  connected  directly 
across  any  convenient  power  source  (say  the  no-  volt  laboratory 
supply)  and  again  when  connected  to  the  same  line  with  the  con- 
denser in  series.  Calling  V\  and  ¥2  these  voltmeter  readings  and 
Rv  the  resistance  of  the  voltmeter  we  can  easily  derive  the  relation 

Vi—  V2 
RC  =  Rv  —  7  —  . 


The  value  of  Re  so  obtained  by  direct  current  measurement 
is  the  leakage  resistance  of  the  condenser,  an  entirely  different 
quantity  from  the  resistance  obtained  in  the  A.C.  test  by  divid- 
ing the  watts  used  in  the  condenser  by  the  square  of  the  current. 
The  leakage  resistance  (or  insulation  resistance)  will  generally 
be  many  megohms  for  one  microfared  of  capacity  whereas  the 
equivalent  series  resistance,  obtained  in  A.C.  test,  will  be  but  a 
few  ohms.  Both  these  resistances  vary  inversely  with  the 
number  of  condensers  connected  in  parallel. 

Calculate  cos  <£,  for  whole  circuit,  for  the  four  different  runs. 
Construct  vector  diagrams  of  the  voltages  across  the  different 
parts  of  the  circuit  and  from  these  construct  the  resultant  voltage  ; 
this  should,  of  course,  equal  the  impressed  voltage,  in  magnitude 
and  phase.  Carry  this  construction  out  for  the  values  obtained 
in  each  of  the  four  sets  of  readings.  On  each  diagram  plot  the 
measured  value  of  impressed  voltage  for  comparison  with  the 
vectorially  obtained  resultant. 


150  TESTING   OF   ELECTRICAL   MACHINERY 

QUESTIONS 

Compare  the  resistances  as  obtained  in  D.C.  test  with  those 
ootained  in  A.C.  test.  Explain. 

What  would  be  the  power  factor  of  the  circuit  if  the  induct- 
ance and  capacity  reactances  were  equal? 

Why  does  the  arithmetical  sum  of  the  watts  used  in  the  dif- 
ferent parts  of  the  circuit  equal  the  total  watts? 

Why  does  not  the  arithmetical  sum  of  the  different  voltages 
equal  the  impressed  E.M.F.? 

The  voltage  across  one  part  of  the  series  circuit  may  be  larger 
than  the  impressed  voltage.  Explain. 

If  an  inductance  of  o.i  henry  and  a  capacity  of  100  microfarads 
are  connected  in  series  with  a  resistance  of  5  ohms  to  a  no- volt 
circuit,  the  voltage  of  which  is  held  constant  while  the  frequency 
is  varied,  at  what  value  of  frequency  will  the  current  be  a  max- 
imum, and  how  much  will  it  be?  What  will  be  the  drop  in 
voltage  across  the  resistance,  condenser,  and  inductance  at 
this  value  of  frequency? 

What  will  be  the  power  factor  and  current  in  the  above  circuit 
when  the  impressed  voltage  has  a  frequency  of  60  cycles? 

What  is  the  reactance  and  what  is  the  impedance  of  the  cir- 
cuit at  this  frequency? 


EXPERIMENT  III 

The  Alternator;  its  Characteristics,  Measured  and  Pre- 
dicted.* The  alternating  current  generator  consists  essentially 
of  a  coil  of  wire  rotating  in  a  magnetic  field,  the  ends  of  the 
coil  being  connected  to  slip  rings  from  which  power  is  taken  by 
means  of  brushes.  Such  a  generator  gives  an  E.M.F.  which  is 
continually  reversing  in  direction.  The  air  gap  is  so  shaped 
and  the  coils  so  placed  on  the  armature  that  the  wave  of  generated 
E.M.F.  is,  as  nearly  as  possible,  a  sine  wave  with  respect  to  time. 

The  magnetic  field  of  the  machine  is  obtained  from  electro- 
magnets which  must  be  excited  from  some  source  of  direct  current 
power.  This  is  one  feature  which  distinguishes  the  A.C.  from 
the  D.C.  generator,  the  latter  being  self  exciting.  In  an  alter- 
nating current  generating  station  several  comparatively  small 
direct  current  machines,  called  exciters,  are  run  merely  to  supply 
the  field  current  for  the  alternators. 

The  E.M.F.  induced  in  an  armature  coil  reverses  every  time 
the  coil  passes  a  field  pole  and  so  makes  a  complete  cycle  for 
every  pair  of  poles  passed.  To  figure  the  frequency  (number 
of  cycles  per  second)  an  A.C.  generator  is  supplying,  it  is  only 
necessary  to  multiply  the  revolutions  per  second  by  the  number 
of  pairs  of  poles. 

For  a  given  strength  of  magnetic  field  and  fixed  speed  the 
generated  voltage  of  an  A.C.  generator  must  remain  constant, 
i.e.,  independent  of  load.  As  load  is  put  on  the  machine  with 
above  conditions  fixed  the  terminal  voltage  will,  however,  decrease, 
the  decrease  being  nearly  proportional  to  the  load  current.  At 
loads  greater  than  rated  value  the  decrease  in  voltage  is  con- 
siderably greater  for  a  given  increase  of  load.  When  the  load 

*  Although  the  following  tests  are  analyzed  from  the  viewpoint  of  single-phase 
apparatus,  we  have  found  it  more  satisfactory  to  carry  out  Ex.  9  after  Ex.  2, 
and  then  perform  all  of  the  machine  tests  with  polyphase  connections  and  loads. 

151 


152  TESTING  OF  ELECTRICAL  MACHINERY 

is  zero  the  terminal  and  generated  voltages  are  equal.  But  when 
current  is  flowing  through  the  armature  windings,  there  occur 
in  the  armature  coils,  reactions  w^hich  must  be  overcome  by  the 
generated  E.M.F.  As  these  reactions  are  proportional  to  the 
current  it  is  evident  that  the  terminal  voltage  will  fall  with 
increasing  load,  the  generated  E.M.F.  remaining  constant.  The 
decrease  of  terminal  voltage  with  load  depends  not  only  upon  the 
amount  of  load,  but  also  upon  the  kind  of  load,  the  decrease 
being  much  greater  for  inductive  than  for  non-inductive  loads. 
The  reason  for  this  will  appear  later. 

The  voltage  of  a  line  to  which  incandescent  lamps  are  con- 
nected should  remain  as  nearly  constant  as  possible.  If  the 
voltage  decreases  the  amount  of  light  given  off  decreases  very 
rapidly,  while  if  the  lamps  are  operated  at  a  higher  than  rated 
voltage,  the  life  is  materially  shortened.  A  decrease  of  5  per  cent 
from  rated  voltage  cuts  down  the  amount  of  light  from  a  carbon 
incandescent  lamp  nearly  30  per  cent;  an  increase  of  voltage 
of  5  per  cent  above  rating  cuts  down  the  life  to  35  per  cent  of  its 
rated  value.  Similar  effects  occur  with  tungsten  incandescent 
lamps  but  not  to  such  a  marked  degree,  as  the  resistance  of 
tungsten  increases  >vith  an  increase  in  temperature,  while  the 
opposite  is  true  of  carbon. 

The  feasibility  of  maintaining  constant  voltage  on  the  line 
when  the  load  is  varied  is  an  important  point  to  investigate. 
That  alternator  giving  the  most  constant  terminal  voltage  with 
varying  load,  will  most  satisfactorily  carry  a  lighting  load  and 
will  require  the  least  field  adjustment  with  change  of  load.  If 
rated  load  is  put  on  an  alternator  and  the  field  current  adjusted 
until  rated  terminal  voltage  is  obtained,  then  maintaining  field 
current  constant,  the  load  is  taken  off,  the  terminal  voltage  will 
rise  and  the  amount  of  this  rise  is  a  measure  of  the  regulation  of 
the  machine.  By  definition, 

_       voltage  at  no  load  —  voltage  at  full  load 
Regulation  (m%)  =—  — r—        .  r  „  , — -\ —        — — . 

voltage  at  full  load 

The  regulation  of  an  alternator  on  non-inductive  load  will  be 


THE  ALTERNATOR 


153 


somewhere  from  5  to  15  per  cent,  depending  upon  the  constants 
of  the  armature  (resistance,  inductance,  etc.). 

The  first  part  of  this  experiment  consists  in  actually  loading 
the  generator,  and,  by  taking  readings  of  terminal  voltage  and 
load  current,  getting  enough  points  to  actually  construct  the 
curve,  showing  the  relation  between  load  current  and  terminal 
voltage,  which  is  called  the  external  characteristic.  For  non- 
inductive  load  a  water  barrel  or  lamp  board  may  be  used;  for 
inductive  load  a  variable  inductance  coil  is .  to  be  connected 
in  parallel  with  the  non-inductive  load.  The  connections  will 
be  as  in  Fig.  14,  in  which  A  represents  the  lamp  board  and  B 
the  variable  inductance.  For  getting  the  external  character- 
istic with  non-inductive  load  switch  C  is  left  open,  and  A,  the 
non-inductive  load,  is  so  varied  that  the  armature  current  is 
adjusted  for  the  desired  values. 

After  the  alternator  is  running  at  rated  speed,  adjust  the  load 
current  and  field  current  so  that  rated  terminal  voltage  is  obtained 
with  rated  current.  Read  the  field  current  and  maintain  it  at 


Lamps  or 
Water  rheostat 


FIG.  14 

this  value  throughout  the  run.  Keeping  speed  constant,  take 
readings  of  armature  current  and  terminal  voltage,  with  values 
of  current  equal  approximately  to  i£,  ij,  rated,  |,  -|,  \  and  zero 
load. 

To  get  the  external  characteristic  with  inductive  load  (corre- 
sponding to  a  commercial  load  of  transformers,  induction  motors, 
etc.),  the  variable  inductance  must  be  used.  The  method  of 
manipulating  the  inductance  coil  is  as  follows.  Suppose  the 
machine  is  rated  at  50  amperes  at  1 10  volts  and  the  power  factor 
desired  is  0.8. 


154  TESTING   OF   ELECTRICAL  MACHINERY 

With  maximum  inductance  in  the  coil,  close  switch  C,  having 
the  voltage  about  normal.  The  watts  necessary  for  full  load 
current  at  .8  power  factor  are  noX5oX.8  =  44oo.  Adjust  the 
lamp  bank  until  the  wattmeter  reads  approximately  4.4  K.W. 
Then  decrease  the  inductance  of  coil  B  until  the  ammeter  A  reads 
about  50  amperes.  Then  adjust  the  field  current  to  bring  the 
terminal  voltage  to  rated  value  and  bring  the  reading  of  the 
ammeter  A  to  exactly  50  amperes,  and  recalculate  the  power 
factor.  A  slight  readjustment  of  loads  A  and  B  will  probably 
be  necessary  to  bring  this  to  the  desired  value.  Time  should 
be  taken  to  adjust  conditions  accurately  for  the  full  load  setting; 
after  the  adjustment  has  been  carried  out  as  carefully  as  is 
feasible,  read  field  current,  armature  current,  terminal  voltage 
and  speed  (which  must,  of  course,  be  at  rated  value).  Keep 
field  current  and  speed  at  these  values  and  proceed  in  similar 
fashion  to  get  points  on  the  external  characteristic  at  about  the 
same  values  as  in  the  previous  run. 

Suppose  for  example  the  half  load  point  on  the  curve  is  to 
be  obtained.  By  inspection  of  the  external  characteristic  for 
non-inductive  load  it  is  found  the  terminal  voltage  was  (let  us 
suppose)  114  volts.  As  we  know,  the  external  characteristic 
for  inductive  loads  rises  more  rapidly  with  decreasing  load  than 
does  that  for  load  of  cos^>=i;  we  assume  that  the  terminal 
voltage  when  the  machine  is  delivering  25  amperes  at  cos  ^>  =  o.8 
will  be,  say,  118.  Then  the  volt-amperes  at  half  load -118X25 
=  2950.  If  the  power  factor  is  to  be  0.8,  the  output  must  be 
2950X0.8  =  2.36  K.W.  So  the  load  A  (Fig.  13)  is  decreased 
until  the  wattmeter  reads  approximately  2.36  K.W.  and  then  the 
inductance  B  is  increased  until  the  current,  as  indicated  in  the 
ammeter,  is  about  25  amperes. 

The  terminal  voltage  is  now  read  and  power  factor  is  checked. 
It  is  probable  that  the  slight  readjustment  of  the  two  load  cir- 
cuits, A  and  B,  will  be  required.  It  requires  a  deal  of  time  to 
set  for  a  certain  power  factor  exactly;  in  this  test  it  is  satisfactory 
if  the  power  factor  is  obtained  within  about  2  per  cent,  that  is, 
if  cos  (f>  is  between  0.78  and  0.82. 


THE    ALTERNATOR 


155 


The  curves  should  look  about  like  those  given  in  Fig.  1 5  and 
should  be  plotted  in  similar  fashion,  the  inductive  character- 


Z.Oflf/ current 

FIG.  15 

istic  being  plotted  in  some  manner  to  distinguish  its  points  from 
the  other  curve. 

To  determine  experimentally  the  external  characteristic  of  an 
alternator  is  always  more  or  less  expensive  (because  of  the  power 
used)  and  frequently  it  is  difficult  to  find  proper  inductances  and 
resistances  for  loading.  The  latter  consideration  is  very  important 
when  the  alternators  are  of  high  capacity  and  voltage.  There- 
fore various  methods  for  predetermining  the  characteristic  have 
been  devised.  One  of  the  simplest  methods  will  be  described 
here. 

When  current  is  flowing  through  the  armature  it  offers  resist- 
ance and  inductance  reactions  to  the  passage  of  the  current. 
For  a  given  generated  E.M.F.  we  can  determine  the  terminal 
voltage  by  subtracting  from  the  generated  E.M.F.  the  two  reac- 
tions in  their  proper  magnitude  and  phase.  The  only  measure- 
ments which  it  is  necessary  to  make,  for  this  scheme  of  pre- 
determining the  characteristic,  are  the  resistance  and  inductance 
reactions  at  some  known  value  of  current  (rated  current  pref- 


156 


TESTING   OF  ELECTRICAL  MACHINERY 


erably).  These  two  reactions,  combined  vectorially  with  the 
rated  terminal  voltage,  give  the  generated  voltage  (as  explained 
in  Ex.  2).  If  from  this  generated  voltage  the  reactions  for  any 
other  value  of  current  are  vectorially  subtracted  the  vector 
remainder  will  be  the  terminal  voltage  for  that  current. 


Source  of 

variable 

A.C.  voHaqe 


AC. 


FIG.  16 

With  the  armature  stationary  and  connections  as  in  Fig.  16, 
impress  enough  A.C.  voltage  (of  frequency  same  as  that  the 
alternator  is  rated  to  give)  to  force  full  load  current  through 
the  armature  with  normal  exciting  current  in  the  field  coils. 
Read  amperes,  volts  and  watts.  The  voltage  necessary  will 
generally  be  about  20  per  cent  of  the  rated  E.M.F.  of  the  machine. 
Take  four  sets  of  readings  with  the  armature  in  different  angular 
positions  with  respect  to  the  field  poles.  Take  the  positions 
about  45°  (electrical)  apart. 

Calculate  the  resistance  reaction  by  dividing  .the  average 
watts  by  current.  Obtain  the  average  impedance  drop  and  so 
calculate  the  average  inductance  reaction  by  the  formula, 


Inductance  reaction  =  Vimpedance  reaction2  — resistance  reaction2 

Referring  to  Fig.  17,  the  method  for  predetermining  the  external 
characteristic  for  non-inductive  load  is  given.  The  rated  terminal 
voltage  OEt  is  laid  off  in  phase  with  the  current  OI.  The  resist- 
ance and  inductance  reactions  are  shown  at  OR  and  OX.  The 
generated  voltage  is  found  as  OEg,  and  the  locus  of  the  E.M.F. 
as  the  load  varies  is  the  circular  arc  through  ^described  with  O  as 
center.  At  half  load  the  reactions  will  be  one-half  as  large  (of 
course  in  same  phase  with  respect  to  current  as  before)  and  the 


THE   ALTERNATOR 


157 


generated  voltage  is  the  same.  The  terminal  voltage  is  therefore 
found  at  OEt' .  At  no  load  the  terminal  voltage  OE"  is  equal 
to  the  generated  voltage  OE0. 

For  inductive  load  the  construction  is  very  similar.     If  the 
power  factor  at  which  the  characteristic  is  desired  is  cos  </>,  the 

\ 

Ea 


R'    R 


FIG.  17 


rated  terminal  voltage  is  laid  off  along  the  line  OA,  making  the 
angle  0  with  the  assumed  phase  of  the  current  01.     In  Fig.  18 


FIG.  18 

the  construction  is  plainly  indicated,  so  that  further  explanation 
is  unnecessary. 

Construct  the  vector  diagrams  for  prediction  of  the  character- 
istics for  loads  of  same  power  factors  as  were  used  in  first  part  of 
test.  Indicate  on  the  curve  sheet  the  points  as  obtained  from 
vector  construction  for  loads  of  approximately  those  used  in  test. 


158  TESTING  OF  ELECTRICAL   MACHINERY 

There  exists  in  alternating  current  generators  another  effect 
which  has  not  yet  been  mentioned;  the  armature  current  has  an 
influence  on  the  field  strength  of  the  alternator,  and  even  if  the 
field  current  be  maintained  constant,  the  generated  E.M.F. 
OEg  does  not  stay  constant  as  the  load  is  varied.  For  this 
reason  the  above  simple  scheme  for  predetermining  the  alterna- 
tor characteristic  is  not  very  accurate,  especially  for  loads  of  low 
power  factors.  In  this  short  course  on  testing,  however,  it  u 
not  feasible  to  employ  one  of  the  more  complicated  methods 
which  do  take  account  of  the  ar.ua.ture  magnetizing  or  demag- 
netizing effect. 

QUESTIONS 

In  this  test  it  is  convenient  tc  use  for  the  inductive  part  of  the 
load,  an  iron  core  inductance  with  a  variable  air  gap.  How 
would  you  expect  the  current  through  the  inductance  to  vary 
with  the  length  of  the  air  gap  and  why  ? 

Assuming  that  the  inductive  circuit  of  the  load  has  a  negligible 
resistance,  how  much  current  is  going  through  each  branch  of 
the  load  circuit  (Fig.  14)  when  the  meters  read  as  follows:  112 
volts,  62  amperes,  5000  watts  ? 

If  the  resistance  of  an  armature  is  o.i  ohm  and  the  reactance 
is  0.3  ohm,  and  the  terminal  voltage  is  125  when  a  non-inductive 
load  of  40  amperes  is  being  supplied,  what  will  be  the  no-load 
terminal  voltage  ? 

With  its  armature  stationary  a  certain  machine  had  its  armature 
connected  to  a  6o-cycle  line  and  the  readings  taken  were:  18 
volts,  50  amperes,  and  325  watts.  What  was  the  impedance  of 
the  armature  ?  Reactance  ?  Self-induction  ?  Resistance  ? 


EXPERIMENT  IV 


The  Transformer;  its  Operating  Characteristics;  Analysis 
of  Losses  and  Predetermination  of  Efficiency.  The  transformer 
is  a  stationary  piece  of  apparatus  by  means  of  which  A.C.  power 
may  be  transformed  from  one  voltage  to  another,  either  higher 
or  lower.  It  finds  its  application  where  A.C.  power  is  to  be 
carried  any  considerable  distance.  For  a  given  size  of  trans- 
mission line  the  power  loss  in  the  line  varies  as  the  square  of 
the  current,  so  that  from  the  standpoint  of  efficiency  the  power 
should  be  at  as  high  a  voltage  as  the  transmission  line  will  safely 
carry.  The  voltage  being  high,  the  current  will  be  correspondingly 
low  and  therefore  the  loss  in  the  line  low.  At  present  it  is  not 
feasible  to  operate  transmission  lines  at  higher  than  140,000 
volts;  if  a  higher  pressure  than  this  is  used  the  losses  by  leakage 
over  insulators  and  actual  leakage  currents  into  the  surrounding 
atmosphere  become  so  great,  that  the  efficiency  of  transmission 
begins  to  decrease  because  of  these  losses. 

A.C.  generators  are  not  economically  built  for  E.M.F.'s 
exceeding  perhaps  15,000  volts.  At  the  point  where  power  is 
used  for  motors,  lights,  etc.,  the  required  voltage  is  generally 
less  than  440  volts.  A  possible  problem  then  is  to  generate  at 
perhaps  10,000  volts,  transmit  at  100,000  volts  and  utilize  at 
200  volts.  To  accomplish  these  changes  in  voltage  is  the  function 
of  the  stationary  transformer.  At  the  beginning  of  the  line  a 
"step  up"  transformer  with  ratio  i  to  10  would  be  used;  at 
the  end  of  the  line  a  "  step  down  "  transformer  of  ratio  500  to  i 
would  be  used.  The  latter  transformation  might  possibly  be 
carried  out  in  two  steps:  the  transmission  line  might  be  con- 
nected to  the  distributing  feeders  with  50:1  transformers,  and 
the  supply  for  lights,  etc.,  be  taken  off  the  distributing  feeders 

159 


160  TESTING   OF   ELECTRICAL   MACHINERY 

by  step  down  transformers  of  ratio  10 :  i.  The  difference  between 
a  "  step  up  "  and  a  "  step  down  "  transformer  is  merely  one 
of  service;  a  10  K.V.A.  noo-no  volt  transformer  is  one  which 
will  transform  10  kilo- volt  amperes  from  noo  to  no  volts  if 
used  as  a  "  step  down  "  transformer  or  will  transform  the  same 
amount  of  power  from  no  volts  to  about  uoo  volts,  when  used 
as  a  "  step  up  "  transformer. 

A  transformer  consists  essentially  of  a  closed  iron  magnetic 
circuit  upon  which  are  wound  two  insulated  coils  of  wire.  The 
two  coils  are  generally  wound  in  sections,  the  different  sections 
being  so  interspersed  that  the  magnetic  leakage  between  the  two 
coils  is  a  minimum.  On  open  circuit  (i.e.,  the  transformer  sup- 
plying no  load)  the  ratio  of  voltages  is  equal  to  the  ratio  of  the 
numbers  of  turns  of  the  two  coils;  as  load  is  put  on  the  trans- 
former the  terminal  voltage  of  the  secondary  will  decrease  slightly 
from  this  value.  The  name  "  secondary "  is  applied  to  that 
coil  from  which  power  is  taken;  the  coil  connected  to  the  power 
supply  line  is  termed  the  "  primary." 

The  operation  of  a  transformer  is  essentially  as  follows: 
When  the  primary  coil  is  connected  to  a  source  of  A.C.  power, 
a  current  will  flow  in  the  coil,  and  if  there  were  no  variation  of 
permeability  in  the  iron  core  this  current  would  be  of  the  same 
form  as  the  E.M.F.  wave  of  the  source  of  supply.  When  varia- 
tion of  the  permeability  occurs  to  any  appreciable  extent  in  the 
iron  core  this  exciting  current  is  distorted  in  form,  but  alternates 
with  the  same  frequency  as  the  impressed  E.M.F.  The  alter- 
nating current  produces  in  the  iron  core  an  alternating  magnetic 
field.  Now  any  other  coil  threading  the  alternating  magnetic 
field  has  induced  in  it  an  alternating  E.M.F.,  the  magnitude  of 
which  E.M.F.  depends  upon  the  number  of  turns  in  the  second 
coil.  The  secondary  E.M.F.  has  the  same  shape  as  the  E.M.F. 
impressed  on  the  primary. 

The  exciting  current  (primary  current  with  secondary  open 
circuited)  is  only  a  few  per  cent  of  the  full  load  rated  current  of 
the  transformer.  When  the  secondary  is  loaded  with  a  certain 
current  a  corresponding  current  flows  into  the  primary  because 


THE  TRANSFORMER  161 

of  the  reactions  occurring  between  the  two  coils,  the  reactions 
being  brought  about  by  the  magnetic  field  which  is  common  to 
the  two  coils. 

If  a  constant  E.M.F.  is  impressed  on  the  primary  the  second- 
ary terminal  volts  decrease  somewhat  with  increase  of  load;  the 
decrease  is  caused  by  the  resistance  and  inductance  reactions 
which  occur  in  the  transformer  itself. 

Three  characteristics  to  be  investigated  in  this  test,  are  effi- 
ciency, primary  power  factor,  and  secondary  terminal  volts, 
the  load  to  be  non-inductive  and  the  primary  impressed  E.M.F. 
to  be  maintained  constant  at  rated  value. 

The  efficiency  will  be  found  to  rise  very  rapidly  with  the  load 
and  will  be  practically  constant  between  J  full  load  and  ij  full 
load.  There  are  no  moving  parts  to  the  transformer  and  hence 
no  mechanical  losses  to  be  supplied.  This  feature  makes  the 
transformer  the  most  efficient  of  all  pieces  of  A.C.  apparatus. 
The  full  load  efficiency  of  a  transformer  may  be  between  92  and 
98  per  cent,  depending  upon  the  capacity,  being  greater  for  the 
larger  sizes. 

At  no  load  the  power  factor  is  very  low,  perhaps  0.3  or  0.4. 
As  load  is  put  on,  this  rapidly  increases  and  from  J  load  up  will 
be  between  .98  and  i.o,  these  figures  being  for  non-inductive 
load  on  the  secondary.  At  no  load  the  secondary  voltage  is 
equal  to  the  impressed  E.M.F.  multiplied  by  the  ratio  of  the  num- 
bers of  turns  on  the  primary  and  secondary.  As  load  is  applied 
the  secondary  terminal  E.M.F.  gradually  decreases,  the  amount 
of  decrease  depending  upon  the  amount  of  load  and  whether  or 
not  it  is  inductive.  For  non-inductive  load  the  decrease  from 
no  load  to  full  load  may  be  about  5  per  cent  or  less. 

In  obtaining  these  three  characteristics  by  loading,  make 
connections  as  in  Fig.  19.  By  the  arrangement  of  two  double 
throw  and  two  single  throw  switches  as  shown,  only  one  set  of 
meters  is  required.  The  most  important  reason  for  using  this 
switching  arrangement,  however,  is  to  get  rid  of  calibration  errors 
in  the  instruments.  The  efficiency  and  power  factor  being  so 
high,  the  different  meters  have  to  be  very  accurate,  if  absurd 


162 


TESTING  OF  ELECTRICAL  MACHINERY 


results  are  not  to  be  obtained  (efficiency  greater  than  100  per  cent, 
etc.).  The  voltmeter  V\  is  used  merely  to  maintain  the  impressed 
voltage  of  the  transformer  constant.  The  input  in  volts,  amperes 
and  watts  is  obtained  by  having  Si  and  S2  thrown  to  the  left, 
54  open  and  53  closed.  To  get  output,  S±  is  first  closed,  Si 
and  £2  thrown  to  the  right,  then  S3  opened.  The  load  is  fixed 
at  the  desired  value,  then  while  V\  is  maintained  at  the  rated 
value  of  the  transformer,  both  input  and  output  are  read. 

As  the  load  is  non-inductive,  the  wattmeter  reading  of  output 
should  equal  volt-amperes  output.     If  it  is  not  so,  a  wattmeter 


Primary 


Secondary 


r 

g 

A.  C. 
Supply 

k 

§ 

T            N 

T 

8 

r 

** 

r 

Non-Inductive 
Load 


FIG.  19 


calibration  curve  is  to  be  constructed,  using  wattmeter  reading 
(of  secondary  load)  as  one  ordinate  and  volts  X  amperes  as  the 
other.  Calling  the  voltmeter  and  ammeter  correct  in  their  cali- 
bration, the  correct  wattmeter  reading  is  thus  given  in  terms  of 
their  readings.  From  this  calibration  curve,  correct  the  readings 
of  input  watts.  The  efficiency  is  then  obtained  as  the  quotient 
of  watts  output  to  watts  input,  corrected  readings  being  used. 
The  power  factor  of  the  primary  is  obtained  as  quotient  of  watts 
(corrected)  by  volt-amperes  of  primary.  The  external  character- 


THE  TRANSFORMER  163 

istic  is  obtained  directly  from  the  readings  of  secondary  E.M.F. 
and  current. 

There  are  two  different  losses  which  occur  in  the  transformer: 
hysteresis  and  eddy  currents  in  the  iron  core,  due  to  the  contin- 
ually reversing  magnetic  field,  and  the  ohmic  or  copper  loss, 
due  to  the  currents  flowing  through  the  windings  of  the  trans- 
former. The  iron  loss  is  practically  independent  of  load  as  the 
strength  of  the  magnetic  field  in  the  core  changes  but  slightly  from 
no  load  to  full  load.  It  is  to  be  regarded  as  constant  in  this  test. 

When  there  is  no  load  on  the  transformer  there  is  flowing 
in  the  primary  coil  merely  the  exciting  current,  the  magnitude 
of  which  varies  with  different  makes  of  transformers  between 
2  and  8  per  cent  of  full  load  current.  In  the  secondary  coil  there 
is  no  current  at  all;  when  it  is  remembered  that  the  copper  loss 
varies  with  the  (current)2  it  is  readily  appreciated  that  the  copper 
loss  of  the  transformer  with  open  secondary  circuit  is  entirely 
negligible.  The  iron  loss  is,  however,  normal.  The  no-load  input 
is  therefore  taken  as  being  all  iron  loss;  the  loss  is  measured 
by  using  a  suitable  wattmeter  in  the  primary  and  impressing 
normal  E.M.F.  at  rated  frequency;  the  secondary  is  to  be  open. 

The  wattmeter  to  be  used  for  obtaining  this  reading  must 
have  a  potential  capacity  equal  to  the  rated  E.M.F.  of  the  trans- 
former and  a  current  coil  of  capacity  equal  to  about  5  per  cent  of 
the  current  rating  of  the  transformer.  This  wattmeter  is  to  be 
used  only  for  the  iron  loss  test;  be  sure  and  remove  it  from  the 
circuit  before  making  the  copper  loss  test  which  is  described  below. 

Because  of  the  retentiveness  of  the  iron  magnetic  circuit  of 
the  transformer,  the  current  which  is  taken  when  the  transformer 
is  first  switched  to  a  line  of  rated  voltage  may  be  excessive  and 
generally  will  cause  damage  to  the  wattmeter.  To  get  rid  of 
this  possibility,  before  connecting  the  transformer  to  the  line 
reduce  the  voltage  of  the  line  supplying  the  power  to  as  low  a 
value  as  possible,  switch  the  transformer  to  the  line  and  then 
gradually  bring  up  the  voltage  to  normal.  Only  one  reading 
of  the  iron  loss  is  necessary. 

The  copper  loss  may  be  put  equal  to  (IP2RP  +  I82RS)  where 


164 


TESTING   OF   ELECTRICAL  MACHINERY 


IP  and  Is  represent  the  primary  and  secondary  currents  and  RP 
and  Rs  represent  the  two  resistances.  If  a  is  the  transformation 
ratio  of  the  transformer  we  must  have  Ip  =  als.  The  secondary 
coil  will  then  have  a  times  as  many  turns  as  the  primary  and  the 
cross  section  of  the  secondary  wire  will  be  i/a  times  as  large 
as  that  of  the  primary  for  the  most  efficient  use  of  the  copper 
wire.  We  therefore  have  Rs  =  a2Rp. 

Therefore  copper  lo$s  =  Ip2Rp+I?Rs 
=  Is2Ri>+L2Rs 

=IS2(RS+RS) 


where  R  is  what  we  may  call  the  equivalent  resistance  of  the 
transformer.  If  the  copper  loss  is  measured  for  any  value  of 
/„  R  can  be  computed.  If  connections  are  made  as  in  Fig.  20 
the  copper  loss  may  be  measured.  With  the  secondary  short 
circuited  only  a  very  small  impressed  E.M.F.  is  necessary  to 
cause  full  load  current  to  flow  in  the  secondary  circuit.  If  then 


D.C. 


FIG.  20 


the  wattmeter  reading  is  taken,  it  represents  the  copper  loss  in 
both  coils  for  full  load  secondary  current.  It  is  well  to  excite 
the  alternator  field  by  a  potentiometer  connection  to  the  D.C. 
line;  the  voltage  may  be  reduced  as  low  as  desirable  with  this 
scheme  of  connections. 

The  wattmeter  used  in  this  test  must  have  a  current  capacity 
equal  to  full  load  current  of  the  primary.  It  will  likely  be  neces- 
sary to  use  a  small  potential  transformer  M  to  get  sufficient  E.M.F. 
on  the  wattmeter  to  give  a  readable  deflection.  The  wattmeter 
reading  must  then  be  divided  by  the  ratio  of  M. 

A  small  iron  loss  is  incurred  in  this  test,  but  it  is  so  small  as 
to  be  negligible.  The  iron  loss  varies  nearly  with  the  impressed 


THE   TRANSFORMER 


165 


E.M.F.  to  the  1.6  power  and  so  is  very  small  when  the  impressed 
E.M.F.  is  small. 

After  R  is  computed  from  the  copper  loss  test,  the  curve  of  PR 
may  be  plotted  by  taking  suitable  values  of  /.  The  two  loss 
curves  have  the  form  given  in  Fig.  21,  and  they  may  be  used  to 
predict  quite  accurately  the  efficiency  *of  the  transformer  for 
different  loads.  The  total  loss  curve  is  plotted  as  the  sum  of 
the  iron  and  copper  losses.  Suppose  the  curves  represent  the 
losses  in  a  transformer  whose  secondary  voltage  is  100  (assumed 
constant  for  this  calculation  with  practically  no  error  involved.) 
At  40  amperes  output,  non-inductive  load,  the  watts  output  = 


750 


I 

cs  100 


10 


20 
Secondary  current 

FIG.  21 


40 


'  Total  loss 


Cu  loss 


Fe  loss 


50 


100X40  =  4000.  From  the  loss  curve  it  is  seen  that  with  40 
amperes  output  the  loss  is  115  watts.  The  input  must  there- 
fore be  =4000  +  115  =  4115  watts.  The  efficiency  =  output/ 

4000 

output    may 


be 


input  =  J .     The  efficiency  for    any    other 

similarly  computed. 

Make  the  load  test  called  for  and  plot  the  three  character- 
istics on  one  curve  sheet  with  current  output  as  abscissa. 


166  TESTING   OF   ELECTRICAL   MACHINERY 

Measure  the  two  losses  as  described ;  plot  the  results  on  a 
second  sheet  and  calculate  the  efficiency  for  several  outputs. 
Plot  the  efficiency  points  so  obtained  on  curve  sheet  No.  i,  to 
see  how  nearly  the  predicted  values  of  efficiency  agree  with  the 
measured  values.  If  discrepancies  occur  it  is  likely  that  the 
predicted  values  are  the  correct  ones,  as  this  prediction  method 
is  more  accurate  than  the  actual  measurement. 

The  other  characteristics  may  also  be  predetermined,  as  was 
the  efficiency,  but  it  is  not  thought  well  to  further  complicate 
the  test. 

QUESTIONS 

A  10  :  i  transformer  has  a  primary  resistance  of  13.1  ohms 
and  a  secondary  resistance  of  0.125  ohm.  What  is  the  equiv- 
alent resistance  of  tfie  transformer  in  terms  of  primary  current  ? 

A  certain  transformer  has  a  primary  rating  of  11,000  volts 
and  100  amperes.  The  exciting  current  is  7  per  cent  of  full 
load  current  and  the  iron  loss  is  16.5  K.W.  What  is  the  no- 
load  power  factor? 

What  are  the  active  and  reactive  components  of  the  exciting 
current?  If  the  copper  loss  of  above  transformer  is  5.5  K.W.  at 
half -load,  what  is  the  full  load  efficiency  ?  What  is  the  equivalent 
resistance  in  terms  of  primary  current  ? 

A  certain  transformer  has  a  rating  of  no  volts,  70  amperes. 
Its  full  load  efficiency  is  96.5  per  cent.  The  input  at  no  load 
(rated  frequency  and  voltage  impressed  on  primary)  is  122 
watts.  What  is  the  copper  loss  at  quarter  load  ? 

A  transformer  is  used  for  supplying  power  to  induction  motors, 
the  average  power  factor  of  which  is  0.70.  What  must  be  the 
rating  (in  kilovolt-amperes)  of  a  transformer  suitable  for  sup- 
plying 65  K.W.  of  power  to  this  load  ? 

A  lo-kilovolt-ampere  transformer  supplies  its  full  rated 
output  to  a  lamp  load  for  2  hours  a  day  and  half  load  for  ij 
hours  a  day.  Its  full-load  copper  loss  is  2  per  cent  of  its  rating 
and  iron  loss  is  ij  per  cent  of  its  rating.  What  is  its  all-day 
efficiency  ? 


EXPERIMENT  V 

The  Induction  Motor;  its  Operating  Characteristics  with  and 
without  Added  Rotor  Resistance.  For  such  purposes  as  require 
a  driver  of  practically  constant  speed,  when  alternating  current 
power  is  available,  the  induction  motor  is  nearly,  always  used. 
Its  speed  is  not  quite  constant,  but  decreases  as  the  load  is 
increased;  the  decrease  in  speed,  or  "  slip,"  as  it  is  called,  may 


Current  in 
phase  #1 


FIG.  22 

be  between  5  and  8  per  cent  at  full  load,  the  slip  being  expressed 
in  percentage  of  synchronous  speed.  A  motor  which  runs  at  1198 
R.P.M.  at  no  load  for  example,  might  run  at  1126  R.P.M. 
at  full  load;  it  would  have  a  slip  of  6  per  cent. 

Nearly  all  induction  motors  are  polyphase,  i.e.,  they  are  fed 
from  a  network  of  conductors,  from  which  currents  of  different 
phases  may  be  taken.  Of  all  polyphase  systems  the  three  phase 
is  most  important,  but  as  the  two  phase  motor  serves  as  well  for 
analysis  as  the  three  phase  and  is  somewhat  simpler  to  represent, 
it  will  be  described  here. 

A  two  phase  generator  is  one  having  usually  two  entirely  sep- 

167 


ies 


TESTING  OF  ELECTRICAL  MACHINERY 


arate  coils.  The  coils  are  identical  in  every  respect  except  their 
position  on  the  armature,  one  coil  being  placed  90°  (electrical) 
behind  the  other.  The  two  coils  are  connected  each  to  two  slip 
rings  and  the  two  phase  power  is  distributed  on  four  wires. 
(Three  rings  and  three  wires  may  sometimes  be  used.)  If  two 
similar  loads  are  connected  to  the  two  phases,  the  currents  in 
these  load  circuits  will  have  the  form  and  phase  relations  given 
in  Fig.  22. 


__^ 

Phdse  1 

I 

/y 

//v*    \ 

7  /  /  * 

)) 

/     / 


Phake 2 


FIG.  23 

Suppose  now  that  we  have  a  two  phase  induction  motor  with 
the  stator  (stationary  part)  wound  with  two  sets  of  coils  on  poles 
as  shown  and  that  the  two  coils  are  connected  to  a  two  phase 
system  as  shown.  By  reference  to  Fig.  23,  the  polarity  of  the 
magnetic  field  of  the  motor  may  be  determined  at  any  time,  and 
in  Fig.  23  this  polarity  is  represented  for  the  different  times 


THE    INDUCTION   MOTOR  169 

shown  in  Fig.  22,  by  the  letters  A7,  5,  O,  on  the  different  circles. 
Circle  numbered  i  shows  the  polarity  of  the  field  at  time  i.  It 
may  thus  be  easily  seen  that  a  magnetic  field  produced  by  two 
windings  90°  apart  and  supplied  with  currents  90°  apart  is 
essentially  a  rotating  one,  the  N  pole  traveling  in  a  clockwise 
direction  in  Fig.  23.  Three  windings  spaced  120°  apart  and 
supplied  with  three  phase  currents  120°  apart  likewise  produce 
a  rotating  magnetic  field.  The  stator  of  an  actual  induction 
motor  is  not  built  exactly  as  shown;  there  are  no  separate  pro- 
jecting pole  pieces;  the  different  windings  are  imbedded  in  slots, 
like  the  winding  of  a  D.C.  armature. 

The  rotor,  or  moving  part,  consists  of  a  laminated  iron  core 
accurately  fitted  to  turn  between  the  poles  of  the  stator.  In  the 
periphery  of  the  rotor  are  imbedded  conductors  which  may  be 
interconnected  in  different  ways.  In  the  squirrel  cage  winding 
the  conductors  are  all  short  circuited  on  one  another  by  being 
connected  to  conducting  end  rings.  Or  the  rotor  may  be 
equipped  with  wound  coils  and  the  ends  of  the  coils  be  connected 
to  slip  rings.  Brushes  bearing  on  these  rings  make  it  feasible 
to  short  circuit  the  coils  if  desired  or  else  the  brushes  may  be 
connected  together  through  resistances,  thereby  increasing  the 
resistance  of  the  rotor  circuit.  This  scheme  of  having  a  wound 
rotor  and  inserting  external  resistance  when  desired  is  used 
in  most  large  size  motors;  the  squirrel  cage  rotor  is  generally  not 
used  in  motors  over  15  H.P.  in  capacity.  Motors  built  for  special 
service,  however,  may  have  §quirrel  cage  rotors  in  sizes  as  high 
as  75  H.P.  or  larger. 

Consider  the  squirrel  cage  rotor  in  the  rotating  magnetic 
field.  If  the  rotor  is  standing  still  and  the  magnetic  field  is 
revolving  it  is  evident  that  an  E.M.F.  will  be  developed  in  the 
rotor  winding  and  as  the  winding  is  short  circuited,  a  current 
will  flow  in  the  rotor  conductors.  But  it  is  a  fundamental 
principle  that  a  conductor  carrying  current  placed  in  a  magnetic 
field  will  be  acted  upon  by  a  force.  By  consideration  of  the 
motion  of  the  field,  direction  of  induced  E.M.F.,  etc.,  it  may  be 
shown  that  the  force  acting  in  the  rotor  conductors  will  be  in  such 


170  TESTING  OF  ELECTRICAL  MACHINERY 

a  direction  that  the  rotor  is  urged  to  revolve  in  the  same  direction 
as  the  magnetic  field.  Just  so  long  as  there  is  relative  motion 
between  the  rotor  and  magnetic  field,  a  torque  will  be  produced 
which  tends  to  accelerate  the  rotor.  If  there  were  no  losses 
of  any  kind  in  the  revolving  rotor  it  would  continue  to  accelerate 
until  the  relative  motion  of  rotor  and  field  was  zero,  i.e.,  the  rotor 
would  turn  at  the  same  angular  speed  as  the  field,  called  synchronous 
speed;  the  slip  is  then  zero.  There  always  exists  some  brush 
friction  and  windage  to  overcome,  so  that  it  is  always  necessary 
for  the  rotor  to  exert  some  torque,  therefore  the  rotor  never  quite 
reaches  synchronous  speed ;  at  no  load  the  slip  may  be  between 
.2%  and  1.0%. 

Now  as  the  rotor  is  called  upon  to  exert  more  torque,  more 
current  must  flow  in  the  rotor  conductors;  this  can  only  occur 
if  a  greater  E.M.F.  is  induced  in  them,  which  in  turn  requires  an 
increase  in  the  slip.  The  slip  must  therefore  increase  with  load; 
for  small  loads  the  slip  and  load  are  nearly  proportional. 

The  effect  of  increasing  the  rotor  resistance  is  to  increase 
the  slip  necessary  to  exert  a  certain  torque.  The  reason  for  this 
is  almost  self  evident.  To  exert  a  certain  torque  requires  a 
certain  current  in  the  rotor;  but  if  the  resistance  of  the  rotor 
circuit  is  increased,  the  E.M.F.  must  be  correspondingly  increased 
to  produce  the  required  current.  The  E.M.F.  can  only  increase 
by  an  increase  in  slip.  The  increase  in  slip  for  a  certain  torque, 
by  increase  of  the  rotor  circuit  resistance,  is  only  obtained 
by  a  decrease  in  efficiency.  Whatever  heat  is  generated  in  the 
external  resistance  added  to  the  rotor  circuit,  is  just  so  much  loss, 
as  it  is  useless  in  producing  turning  effort  in  the  motor. 

The  maximum  torque  which  an  induction  motor  can  exert 
is  independent  of  the  variations  of  the  rotor  resistance.  But  the 
speed  at  which  this  maximum  torque  occurs  decreases  with  in- 
crease of  resistance.  Therefore  the  "  pull  out  "  point,  or  maxi- 
mum output  of  the  motor,  decreases  with  addition  of  external 
resistance  to  the  rotor  circuit. 

The  power  factor  of  the  induction  motor  is  very  low  at  light 
loads,  increases  with  load  up  to  about  rated  load,  and  then  decreases 


THE  INDUCTION  MOTOR 


171 


again  with  overload.  It  may  be  about  30  per  cent  at  no  load 
and  rise  to  between  80  and  93  per  cent  for  maximum  value,  the 
higher  figure  being  for  large  size  motors. 

At  standstill  the  induction  motor  is  essentially  a  sliort-cir- 
cuited  transformer,  the  rotor  corresponding  to  the  secondary 
o."  a  transformer.  But  if  a  short-circuited  transformer  is  con- 
nected to  a  line  of  normal  voltage  the  current  taken  is  excessive, 
being  perhaps  20  or  30  times  full  load  value.  Because  of  magnetic 
l:a,kage  between  rotor  and  stator  the  conditions  in  the  induction 
motor  at  starting  (rotor  at  standstill)  are  not  quite  so  bad  as 


Phase  2 


Phase  J 


Starting  resistance 


FIG.  24 


with  an  ordinary  transformer.  However,  the  starting  current 
when  an  induction  motor  is  connected  to  a  line  of  normal  voltage 
is  excessive  and  it  is  the  principal  object  of  the  added  external 
rotor  resistances  to  limit  this  starting  current. 

Another  effect  of  introducing  resistance  in  the  rotor  circuit 
at  starting  is  to  increase  the  starting  torque.  An  induction  motor 
exert  its  maximum  torque  when  the  rotor  reactance  is  jusl  equal 
to  the  rotor  resistance.  The  rotor  reactance  varies  directly 
with  the  slip  and  at  standstill  is  much  greater  than  the  resist- 
ance in  the  ordinary  rotor  winding.  By  inserting  extra  resist- 
ance at  starting,  the  resistance  is  brought  equal  to  the  reactance 
and  maximum  torque  is  exerted  at  starting,  a  very  favorable 
condition  when  the  motor  has  to  start  line  shafting,  etc.  As  the 


172 


TESTING  OF  ELECTRICAL  MACHINERY 


motor  speeds  up  the  extra  resistance  is  cut  out  in  steps  and  if 
properly  done,  the  equality  of  resistance  and  reactance  is  nearly 
maintained  as  the  motor  speeds  up;  thus  the  rotor  may  be  made 
to  exert  approximately  its  maximum  torque  all  the  time  during 
which  it  is  accelerating. 

With  a  polyphase  induction  motor  three  runs  are  to  be  made, 
one  with  the  rotor  short  circuited  and  two  with  additional  resist- 
ance in  the  rotor  circuit.  If  a  two  phase  motor  is  used  make 
connections  as  in  Fig.  24,  and  if  three  phase,  as  in  Fig.  25.  Note 
the  effect  on  the  direction  of  starting  of  reversing  the  connections 
of  one  phase;  of  reversing  two  phases. 


To  Starting 
resistance 


FIG.  25 

Instead  of  using  two  sets  of  instruments  as  shown  in  above 
figures,  a  single  set  of  instruments  may  be  used  with  the  combination 
of  switches  given  in  Ex.  4. 

In  measuring  three-phase  power  with  two  wattmeters  as  above, 
one  wattmeter  will  read  negative  if  the  power  factor  of  the  motor 
is  less  than  .5.  Begin  the  run  at  full  load  and  connect  the  watt- 
meters so  that  positive  readings  are  obtained  on  both  phases; 
at  full  load  the  power  factor  will  surely  be  greater  than  .5.  As 
the  load  decreases  one  wattmeter  reading  will  decrease  faster 
than  the  other,  will  reach  zero  at  perhaps  one-quarter  load  and 
at  lighter  load  will  deflect  backward.  Under  this  condition  the 


THE   INDUCTION   MOTOR 


173 


potential  coil  is  to  be  reversed,  reading  taken  and  called  negative, 
i.e.,  the  motor  input  is  the  difference  of  the  two  wattmeter  read- 
ings. No  such  difficulty  will  be 'encountered  with  the  two-phase 
iour-wire  systems  given  in  Fig.  24,  but  may  arise  if  a  three-wire 
two-phase  system  is  used  for  power. 

Calculate  the  full  load  torque  of  the  motor  (assuming  no  slip) 
and  take  readings  of  input,  torque  and  speed  for  about  eight 
values  of  torque  between  zero  and  50  per  cent  overload.  Keep 
impressed  voltage  and  frequency  at  rated  values  of  the  motor. 


1200 


woo 


300 


H.P.  Output 
FIG.  26 


Make  this  run  with  the  rotor  short  circuited  and  make  similar 
runs  with  two  different  values  of  added  resistance  in  the  rotor 
circuit.  The  greater  value  of  resistance  should  be  about  suffi- 
cient to  give  the  rated  full  load  torque  at  half  synchronous  speed ; 
the  other  resistance  to  be  about  one-half  this  value. 

The  curves  obtained  from  test  should  look  similar  to  those  of 
Fig.  26,  which  were  obtained  from  a  10  H.P.,  60  cycle,  220  volt, 
2  phase  motor. 


174 


TESTING  OF  ELECTRICAL    MACHINERY 


The  next  characteristic  of  the  motor  to  be  investigated  is  the 
relation  of  starting  torque  and  current  to  the  resistance  of  the 
rotor  circuit.  Apply  one-half  rated  voltage  (at  rated  frequency) 
to  the  motor,  having  the  rotor  locked.  Take  readings  of  torque 
and  current  for  the  different  points  of  the  starting  resistance. 

Find  out  how  much  starting  torque  a  single-phase  induction 
motor  has ;  this  may  be  tried  by  opening  one  of  the  lines  supplying 
the  power  to  the  stator.  The  stator  will  then  be  supplied  with 
single-phase  power  only,  and  it  will  be  found  that  under  such 


Starting  characteristics 


Rotor  circuit  resistance 

FIG.  27 

conditions  the  motor  exerts  no  starting  torque  whatsoever.  If, 
however,  the  motor  is  allowed  to  reach  normal  running  speed 
while  supplied  with  polyphase  power  and  then  one  line  is  opened, 
it  will  be  found  that  the  motor  will  run  all  right  and  will  carry 
nearly  full  load  before  the  "  pull  out  "  point  is  reached. 

Measure  resistance  of  rotor  and  of  the  various  steps  of  the 
starting  resistance,  if  they  are  not  given.  Plot  curves  of  starting 
torque  and  current  against  rotor  circuit  resistance.  The  results 
should  be  somewhat  of  the  form  of  those  given  in  Fig.  27,  which 
are  for  the  same  motor  as  the  curves  of  Fig.  26. 


THE    SYNCHRONOUS   MOTOR  175 

QUESTIONS 

What  will  be  the  no-load  speed  of  a  6-pole  6o-cycle  induction 
motor?  Of  a  4-pole,  25-cycle  motor?  What  would  be  the 
approximate  full-load  speeds  of  these  two  motors,  assuming  no 
resistance  is  added  to  the  rotor  circuit  ? 

Could  an  induction  motor  be  obtained  from  a  manufacturing 
company,  which  would  give  a  no-load  speed  of  1000  R.P.M. 
when  operated  on  a  6o-cycle  line?  How  many  poles  would  it 
have? 

If  a  6o-cycle,  8-pole  motor  is  made  to  give  a  full-load  speed 
of  4.50  R.P.M.  by  added  rotor  resistance,  about  what  efficiency 
might  be  expected  ? 

If  a  three-phase  induction  motor  has  a  fuse  blown  in  one  line, 
how  much  starting  torque  will  the  motor  exert  ? 

If  the  brushes  on  the  slip  rings  of  a  wound  rotor  induction 
motor  are  lifted,  how  much  torque  will  the  motor  exert?  Why? 

A  6o-cycle  motor  has  a  slip  at  full  load  of  8  per  cent  The 
self-induction  of  a  rotor  coil  is  o.ooi  of  a  henry.  The  resistance 
of  the  rotor  coil  is  0.05  ohm.  What  is  the  impedance  of  the 
coil  at  full  load  ?  At  standstill? 


EXPERIMENT  VI 

The  Synchronous  Motor;  Phase  Characteristics  and  Phase 
Shifting  with  Load.  If  two  alternating  current  generators  are 
operating  in  parallel,  supplying  power  to  the  same  line,  and  the 
driving  power  is  taken  away  from  one  of  them,  it  will  ordinarily 
continue  to  run  at  exactly  the  same  speed  it  had  before  (provided 
the  other  alternator  does  not  slow  down)  and  it  draws  from  the 
other  alternator  the  power  necessary  to  run  itself.  An  alter- 
nator so  running  is  termed  a  synchronous  motor,  as  it  runs  exactly 
in  synchronism  with  the  alternator  supplying  its  power.  This 
point  is  to  be  emphasized;  a  6-pole  synchronous  motor  running 
from  a  6o-cycle  power  line  runs  at  1200  r.p.m.  no  load  and  1200 
r.p.m.  full  load,  and  continues  to  run  at  1200  r.p.m.  as  the  load 
is  further  increased,  until  the  "  pull  out  "  point  of  the  motor  is 
reached  at  perhaps  50  per  cent  overload;  when  this  load  is  reached 
the  motor  pulls  out  of  synchronism  with  the  line  and  stops.  As 
soon  as  it  pulls  out  of  step  it  draws  an  excessive  current  and  the 
protective  apparatus  either  at  the  motor  or  in  the  generating 
station  must  open  if  disastrous  results  are  not  to  be  incurred. 

A  synchronous  motor  is  generally  not  self-starting;  some 
auxiliary  driving  power  must  bring  it  up  to  synchronous  speed 
and  when  the  proper  conditions  are  reached  the  switch  connect- 
ing the  motor  to  the  supply  line  is  closed.  A  polyphase  synchron- 
ous motor  is  sometimes  started  as  an  induction  motor,  in  which 
case,  of  course,  no  auxiliary  starter  is  necessary.  In  this  method 
of  starting  the  field  is  left  unexcited,  the  armature  is  connected 
to  some  low  voltage  taps  (generally  50  per  cent  normal  or  less) 
on  the  supply  transformers.  The  armature  draws  a  rather 
heavy  current,  perhaps  100  per  cent  in  excess  of  full  load,  and 
this  armature  current  induces  eddy  currents  in  the  pole  faces. 
The  interaction  of  the  armature  current  and  these  eddy  currents 

176 


THE  SYNCHRONOUS  MOTOR  177 

tend  to  make  the  armature  rotate.  It  will  continue  to  accelerate 
until  it  reaches  synchronous  speed;  the  armature  is  then  connected 
to  the  normal  voltage  taps  of  the  supply  transformers  and  the 
field  gradually  excited  until  normal  field  current  is  reached.  Tn 
revolving  field  synchronous  motors  where  this  method  of  starting 
is  used,  each  pole  face  of  the  machine  is  pierced  by  a  number  of 
brass  or  copper  rods  placed  parallel  to  the  shaft;  the  ends  of 
these  rods  are  connected  to  brass  short  circuiting  rings.  The 
machine  thus  starts  as  a  squirrel  cage  induction  motor. 

The  connection  of  the  armature  to  one-half  voltage  taps  and 
then  to  normal  voltage  taps  is  accomplished  by  a  double  throw 
switch;  the  transition  is  made  as  quickly  as  possible  so  that  the 
motor  has  no  time  to  slow  down  during  the  process. 

The  disadvantage  of  this  method  of  starting  is  the  large 
storting  current  taken  from  the  line  at  very  low  power  factor; 
the  resultant  fluctuations  in  the  line  voltage  may  seriously  inter- 
fere with  the  operation  of  other  synchronous  apparatus  connected 
to  the  line;  in  fact,  if  the  fluctuations  in  line  voltage  are  large, 
other  synchronous  apparatus  may  actually  fall  out  of  step.  In 
spite  of  these  disavantages  the  method  is  a  very  common  one  for 
starting  revolving  field  synchronous  motors. 

Another  method  for  starting  a  synchronous  motor  is 
to  use  a  small  induction  motor,  the  rotor  of  which  is  mounted 
on  the  extended  shaft  of  the  synchronous  motor.  The  induction 
motor  may  be  10  per  cent  the  size  of  the  synchronous  motor; 
it  must  have  at  least  one  pair  of  poles  less  than  the  synchronous 
motor;  and  it  is  so  designed  that  when  its  load  is  equal  to  the  no 
load  losses  of  the  synchronous  motor  (core  loss  and  friction)  it  is 
turning  the  synchronous  motor  at  synchronous  speed.  After 
the  synchronous  motor  is  connected  to  the  line,  power  is  cut  off 
from  the  induction  motor  and  the  rotor  runs  idle. 

The  switch  connecting  the  armature  of  the  synchronous  motor 
to  the  supply  line  is  called  the  synchronizing  switch.  Before 
this  may  be  closed  four  conditions  must  be  fulfilled,  unless  the 
induction  motor  principle,  as  described  above,  is  used  for  starting 
the  motor.  These  conditions  are: 


178  TESTING  OF   ELECTRICAL  MACHINERY 

1.  Motor  voltage  equal  to  line  voltage. 

2.  Motor  frequency  equal  to  line  frequency. 

3.  Phase  of  motor  voltage  exactly  opposite  to  that  of  line. 

4.  Wave  form  of  motor  E.M.F.  must  be  nearly  similar  in 
form  to  wave  of  line  E.M.F. 

The  first  three  conditions  may  be  adjusted  by  the  operator, 
the  fourth  is  satisfied  or  not  when  the  machine  is  built.  As  was 
mentioned  in  the  discussion  of  the  alternator,  the  wave  form  of 
an  A.C.  machine  is  determined  by  the  shape  of  air  gap  and  pole 
piece  and  the  distribution  of  the  armature  windings. 

By  analyzing  the  four  conditions  named,  it  is  seen  that  they 
.demand  that  the  motor  voltage  shall  at  every  instant  be  equal 
and  opposite  to  the  line  voltage.  If  that  were  not  the  case  a 

Lamp 


J.C.  Supply  line          *\  fTS)      P  D'C 


Lamp 

FIG.  28 

heavy  current  would  flow  through  the  armature  due  to  the  un~ 
balanced  voltage;  the  current  will  be  large  even  for  a  small  un- 
balancing of  voltage,  because  of  the  low  impedance  of  the  armature. 

The  first  two  conditions  can  be  satisfied  by  the  use  of  a  volt- 
meter and  speed  counter  when  frequency  of  line  supply  is  known. 
To  determine  the  third  condition,  synchronizing  lamps  or  a  syn- 
chronoscope  must  be  used.  The  synchronoscope  is  an  indicating 
instrument  having  two  internal  circuits,  one  of  which  is  connected 
to  the  line  and  one  to  the  motor.  The  position  of  the  pointer 
indicates  the  relative  phases  of  the  two  E.M.F. 's. 

Lamps  are  to  be  used  in  this  test,  so  their  action  will  be  more 
fully  described.  One  lamp  is  connected  across  each  blade  of 
the  synchronizing  switch  A,  as  in  Fig.  28.  The  lamps  will 
flicker  as  the  motor  phase  changes  with  respect  to  the  line  phase. 
They  complete  a  circuit  consisting  of  the  motor  armature,  the 


THE  SYNCHRONOUS  CONVERTER  179 

line  and  armature  of  the  generator  supplying  the  line.  In  this 
circuit  there  are  acting  two  E.M.Fs.,  that  of  the  motor  and  that 
of  the  generator.  At  any  instant  the  effective  voltage  causing 
current  to  flow  through  the  lamps  is  the  resultant  of  the  two; 
as  their  relative  phase  changes,  the  resultant  changes  and  has  for 
its  locus  a  circle  as  shown  in  Fig.  29.  The  resultant  voltage 
for  any  position  of  Em  is  seen  to  be  OR,  the  chord  to  the  circle 
from  the  point  0.  The  current  through  the  lamps,  and  therefore 
their  brilliancy,  will  increase  as  the  motor  voltage  Em  catches  up 
(in  phase)  with  Ea  and  will  decrease  as  the  two  E.M.Fs.  separate 
in  phase.  There  will  be  no  current  through  the  lamps  when  Em 


Err, 

h 

'  / 
s 

X    " 

A 

X 

\ 

// 

\ 

X 

/ 

\ 

\ 

•>< 

FIG.  29 

is  just  opposite  in  phase  to  Eg-,  condition  No.  3  is  therefore 
satisfied  when  the  lamps  are  dark  and  this  is  called  the  "  dark  " 
connection  for  synchronizing.  With  the  lamps  connected  diago- 
nally across  the  switch,  the  proper  time  for  closing  the  switch  is 
at  the  middle  of  a  bright  period. 

After  having  adjusted  the  motor  voltage  to  equal  the  line 
voltage,  the  speed  of  the  motor  is  so  adjusted  (by  adjusting  its 
driving  motor)  that  the  lamps  flicker  once  in  4  or  5  seconds; 
the  switch  A  is  then  closed  in  the  middle  of  a  dark  or  bright 
period,  according  to  the  connection  of  the  lamps.  The  power 
supplying  the  starting  motor  may  then  be  cut  off. 


180 


TESTING  OF  ELECTRICAL    MACHINERY 


If  a  three  phase  motor  is  used,  it  is  necessary  to  use  three 
synchronizing  lamps,  one  connected  across  each  pole  of  the  three 
pole  synchronizing  switch.  The  lights  must  flicker  together; 
if  they  are  bright  and  dark  in  rotation  instead  of  simultaneously, 
one  of  the  phases  is  incorrectly  connected  and  two  of  the  supply 
lines  must  be  reversed  in  their  connection  to  the  motor. 

One  of  the  important  features  of  the  synchronous  motor  is 
well  shown  by  the  set  of  curves  called  "  phase  characteristics." 

If  the  load  on  a  synchronous  motor  is  kept  constant  and  the 


=  No  load 
=  Half  load 
3  =  Full  load 


Field  Current 

FIG.  30 


field  current  varied,  the  armature  current  is  caused  to  vary  in 
about  the  fashion  indicated  in  Fig.  30.  It  is  this  curve,  drawn 
between  armature  current  and  field  current  as  variables,  which, 
is  called  the  phase  characteristic.  For  any  load  a  certain  definite 
field  current  gives  a  minimum  armature  current.  This  value  of 
field  current  gives  the  so  called  normal  excitation.  For  any  value 
of  field  current  other  than  normal  the  armature  current  is  greater 
than  that  taken  with  normal  excitation. 

Now  as  the  load  of  the  motor  is  supposed  constant  the  powei 
component  of  the  armature  current  must  remain  practically  con- 


THE  SYNCHRONOUS  MOTOR 


181 


stant.  It  must  be,  therefore,  that  any  other  than  normal  excita- 
tion on  the  motor  produces  in  the  armature  a  wattless  or  reactive 
current,  which,  being  90°  out  of  phase  with  the  motor  voltage, 
represents  no  expenditure  of  power  in  the  motor.  In  fact  an 
overexcited  synchronous  motor  draws  a  leading  current  from 
the  supply  line  and  an  underexcited  motor  a  lagging  current, 
as  shown  in  Fig.  31.  OE  is  the  phase  of  the  line  voltage  and 
OI  is  the  armature  current  for  no  load  with  excitation  OC,  Fig. 
30.  Excitations  OA  and  OB  result  in  armature  currents  All, 
and  £/2,  Fig.  30,  which  are  represented  vectorially  in  Fig.  31; 
as  OIi  and  O/2. 

If  the  speed  of  a  synchronous  motor  is  independent  of  load 


FIG.  31 


the  question  arises,  how  does  the  motor  adjust  itself  to  take  more 
or  less  load  ?  The  phase  shifting  of  the  motor  armature  is  what 
accomplishes  this  end. 

The  E.M.F.  causing  current  to  flow  through  the  armature 
of  the  motor,  is  the  vector  resultant  of  the  motor  voltage  and  line 
voltage.  This  resultant  varies  with  the  variation  of  the  phase 


182  TESTING   OF   ELECTRICAL   MACHINERY 

difference  of  the  two  E.M.Fs.  In  Fig.  32  are  shown  two 
positions  of  Em,  the  motor  voltage,  with  respect  to  a  fixed  line 
voltage  Et.  The  resultant,  OEr,  is  the  E.M.F.  causing  current 
to  flow  in  the  motor  and  this  changes  widely  with  a  small  shifting 
of  OEm.  The  current  OI  is  equal  to  the  voltage  OEr  divided 
by  the  armature  impedance.  It  is  laid  off  behind  OEr  by  the 
angle  6,  where 

,,     armature  reactance 

tan  0  =  —          . 

armature  resistance 

It  is  variation  of  the  angle  a,  giving  the  phase  position  of 
the  armature,  which  permits  the  motor  to  take  more  or  less, 
load. 

The  variation  in  a  may  be  easily  determined.  Suppose  that 
two  insulated  discs  with  metallic  strips  (as  used  in  Ex.  i)  are 
placed  one  on  the  shaft  of  the  generator  supplying  the  power  and 
one  on  the  shaft  of  the  motor,  and  that  a  voltmeter  is  wired  to 


FIG.  32 

a  1 10- volt  circuit  through  these  two  discs  and  their  brushes 
(Fig-  33)-  The  voltmeter  will  indicate  only  if  the  two  brushes 
are  in  contact  with  the  metal  strips  at  the  same  time.  The  brush 
on  the  motor  moves  over  a  graduated  arc  so  that  its  position 
may  be  read  and  the  brush  on  the  generator  remains  stationary. 
The  position  to  which  the  motor  brush  must  be  moved  before 
the  voltmeter  gives  maximum  deflection  gives  the  phase  position 
of  the  motor  armature  with  respect  to  the  generator  armature. 
If  the  generator  is  large  compared  to  the  motor,  its  armature 
position  is  also  a  measure  of  the  phase  of  the  E.M.F.  impressed 


THE    SYNCHRONOUS    MOTOR 


183 


on  the  motor.     It  will  be  found  that  the  value  of  a  is  almost 
directly  proportional  to  the  load. 

With  the  connections  as  in  Fig.  34,  obtain  the  phase,  charac- 
teristics of  the  motor  for  no  load,  J  load  and  full  load.  On  each 
curve  get  about  8  points,  using  for  armature  current  not  more 
than  150%  of  the  rated  current  of  the  motor;  i.e.,  in  Fig.  30,  OF 
should  not.  be  greater  than  50%  above  the  rating  of  the  motor.  The 
motor  may  be  loaded  by  Prony  brake  or,  what  is  more  convenient, 
by  a  D.C.  generator.  The  generator  may  be  used  as  a  starting 


Voltmeter 


110  Volt 
D.C.  Line 


Generator 


Motor 


FIG.  33 


motor  until  the  A.C.  motor  is  synchronized  and  then  it  may  be 
used  as  generator  to  load  the  synchronous  motor. 

In  getting  the  phase  characteristics  it  is  not  necessary  to  know 
the  exact  load  on  the  motor;  J  rated  current  may  be  used  as  half 
load,  etc. 


FIG.  34 

The  impressed  voltage  and  frequency  are  to  be  held  constant; 
read  volts,  amperes,  watts  input  and  field  current. 


184  TESTING   OF   ELECTRICAL  MACHINERY 

Then  adjust  the  field  current  to  the  value  for  minimum 
armature  current  at  no  load.  Adjust  the  brush  on  the  motor 
disc  until  the  voltmeter  shows  maximum  deflection,  and  read 
brush  position.  Read  also  impressed  E.M.F.  and  field  current, 
which  are  to  be  maintained  constant,  and  also  watts  and  amperes 
supplied  to  armature. 

Put  on  load  in  about  8  steps  from  no  load  to  50  per  cent 
overload,  taking  same  readings  as  for  no  load. 

Plot  on  one  sheet,  against  field  current  as  abscissa,  watts 
input,  amperes  input  and  power  factor  for  each  of  the  runs. 

Plot  on  a  second  curve  sheet,  watts  input  to  motor  (armature 
input  only)  against  brush  position  as  abscissa.  Note  whether 
the  brush  is  moved  with  or  against  the  direction  of  rotation  as 
load  is  increased,  and  so  determine  whether  the  motor  armature 
is  advanced  or  retarded  in  phase  as  the  load  on  the  motor  is 
increased. 

QUESTIONS 

If  a  line  shaft  is  to  be  driven  by  a  direct  connected  synchronous 
motor,  what  speeds  are  ordinarily  obtainable  if  the  power  supply 
is  6o-cycle?  If  25-cycle? 

A  single-phase  synchronous  motor  is  drawing  125  amperes 
from  a  2300- volt  line  at  a  power  factor  of  0.70,  leading  current. 
How  much  power  is  being  supplied  to  the  motor?  How  much  is 
the  leading  component  of  the  armature  current? 

A  synchronous  motor  is  to  be  used  as  a  synchronous  con- 
denser to  compensate  for  the  lagging  component  of  current  taken 
by  an  induction  motor  load.  The  induction  motor  load  is  65  am- 
peres at  a  lagging  power  factor  of  0.76;  the  line  voltage  is  6600. 
Assuming  that  it  requires  no  power  to  run  the  synchronous 
condenser,  what  must  be  its  rating  to  just  neutralize  the  lagging 
current  of  the  motor  load?  What  must  be  its  rating  if  the  power 
factor  of  the  line  is  to  be  increased  from  0.76  to  0.95?  With  the 
latter  condition  what  will  be  the  current  taken  by  the  motor  if 
it  is  also  required  to  deliver  mechanical  energy  as  well,  at  the 
rate  of  200  horse-power  (assuming  motor  efficiency  is  90  per  cent)  ? 


EXPERIMENT  VII 

The  Rotary,  or  Synchronous,  Converter;  Effect  of  Voltage  and 
Speed  upon  Ratio;  Operating  Characteristics.  The  efficient 
transmission  of  electric  power  over  any  considerable  distance 
requires  the  use  of  alternating  currents,  as  explained  in  Ex.  4. 
Practically  all  motors  for  driving  electric  trains  are  D.C.  motors; 
although  alternating  current  motors  have  been  used  in  some 
few  cases,  the  field  of  electric  traction  in  America  is  so  exclusively 
controlled  by  the  direct  current  motor  that  from  the  general 
standpoint  the  A.C.  motor  need  not  be  considered.  The  char- 
acteristics of  a  small  A.C.  series  motor  are  given  in  Fig.  35. 
Also  in  dotted  lines  are  given  the  characteristics  of  the  same 
motor  run  with  direct  current  power;  the  superiority  of  its 
behavior  as  a  D.C.  motor  is  so  evident  that  nothing  further  need 
be  said  regarding  the  comparative  merits  of  A.C.  and  D.C.  rail- 
way motors. 

The  electric  railway  generating  station  is  always  located 
where  water  and  coaling  facilities  are  good,  and  A.C.  power  is 
generated.  This  power  is  transmitted  at  a  high  voltage  to  a 
substation  where  the  voltage  is  stepped  down.  It  is  the  function 
of  the  rotary  converter  to  change  this  low  voltage  A.C.  power 
into  D.C.  power  for  use  in  the  car  motors. 

An  elementary  explanation  of  the  performance  of  a  rotary 
converter  maybe  given  by  supposing  that  the  winding  of  a  D.C. 
shunt  motor  is  tapped  (on  the  end  opposite  to  the  commutator) 
at  two  points  180°  (electrical)  apart.  These  two  taps  are  to 
be  connected  to  two  slip  rings.  Now  any  winding  revolving  in 
a  magnetic  field  generates  an  alternating  E.M.F.  If  the  winding 
is  connected  to  slip  rings  an  alternating  current  is  delivered  to 

185 


186 


TESTING  OF  ELECTRICAL  MACHINERY 


the  outside  circuit;  if  the  winding  is  connected  to  a  commutator 
the  delivered  current  will  be  direct.  From  this  it  follows  that 
the  shunt  wound  motor  would  have  on  its  slip  rings  an  alternating 
E.M.F.;  if  the  rings  are  connected  to  an  external  circuit  an 
alternating  current  will  flow  from  the  A.C.  brushes  and  a  cor- 
responding increase  in  the  D.C.  input  will  occur.  Such  a  machine 
converts  D.C.  power  into  A.C.  power;  it  is  ordinarily  termed  an 


30 


20 


10 


I 

0 


10  100 
9  90 
8  80 
7  70 
6  60 
50 
40 
30 
20 
10 
0 


Efficiency 


30 


40 


50 
Current 

FIG.  35 


60 


70 


80 


"  inverted  "  rotary.  If  now  the  function  of  the  machine  is  reversed, 
i.e.,  A.  C.  power  is  supplied  to  the  slip  rings,  the  machine  run- 
ning as  a  synchronous  motor,  and  D.C.  power  is  taken  from  the 
commutator  end,  it  will  be  operating  as  a  rotary  converter  normally 
does.  The  amount  of  A.C.  power  input  depends  upon  the  amount 
of  D.C.  power  output. 

The  efficiency  of  a  rotary  varies,  of  course,  with  the  size, 
being  perhaps  9$  per  cent  in  the  larger  machines  and  85  —  90  per 
cent  in  the  smaller  ones. 


THE  SYNCHRONOUS  CONVERTER 


187 


The  distinctive  feature  of  a  rotary  of  the  ordinary  type  is  the 
ratio  of  A.C.  to  D.C.  volts.  For  a  single  phase  machine  it  is 
.707,  for  a  three-phase  machine  .612;  it  is  a  constant  which 
depends  only  on  the  number  of  A.C.  taps.  For  a  given  impressed 
A.C.  voltage  the  D.C.  voltage  is  nearly  independent  of  load;  it 
decreases  somewhat  with  increase  of  load  because  of  the  imped- 
ance drop  in  the  armature  windings. 

The  power  factor  of  the  A.C.  input  will  depend  upon  the 
value  of  the  field  current;  in  this  respect  the  rotary  is  exactly 
similar  to  the  synchronous  motor.  Under  ordinary  conditions 


Coil 
/'midway 

between 
Taps 


A.C.  Taps 


FIG.  36 

the  field  current  should  be  adjusted  to  that  value  which  gives  a 
minimum  armature  current  for  the  load  being  carried ;  this  value 
of  field  current  does  not  change  much  with  the  load;  if  adjusted 
for  one  load  it  will  be  nearly  correct  for  any  other. 

The  coils  of  the  armature  carry  the  instantaneous  difference 
of  the  A.C.  and  D.C.  currents,  the  form  of  the  current  flowing 
in  any  one  coil  being  displaced  sections  of  a  sine  curve.  The 
form  of  the  current  wave  in  the  coils,  one  close  to  the  tap  and 
one  half  way  between  taps  is  shown  in  Fig.  36.  The  peculiar 
shape  of  these  current  waves  results  in  unequal  heating  in  the 
different  coils  of  a  rotary,  those  nearest  the  taps  getting  hottest. 


188  TESTING   OF  ELECTRICAL  MACHINERY 

Because  of  the  fact  that  the  B.C.  and  A.C.  currents  in  the 
different  coils  tend  to  neutralize  one  another  the  capacity  of  a 
given  machine  is  considerably  greater  run  as  a  rotary  than  as  an 
alternating  or  direct  current  generator.  A  certain  size  machine 
used  as  generator  or  rotary  has  capacities  as  follows: 

B.C..  generator  capacity  =  100     K.W. 

Single  phase  A.C.  generator  capacity •=   70.7  K.W. 
Single  phase  rotary  capacity  =   84.8  K.W.; 

Three  phase  rotary  capacity  -=133.8  K.W.; 

Six  phase  rotary  capacity  =193.7  K.W. 

The  above  values  are  for  power  factor  of  one.  For  other 
power  factors  the  values  are  somewhat  different. 

Because  of  the  fact  that  the  ratio  of  B.C.  to  A.C.  voltage 
is  fixed  for  any  given  rotary,  it  might  seem  that  a  rotary  could 
not  be  compounded,  i.e.,  give  a  B.C.  voltage  increasing  with 
load.  In  one  way  this  is  true;  for  a  given  impressed  A.C.  voltage 
the  B.C.  voltage  cannot  be  increased,  but  in  practice  the  impressed 
A.C.  voltage  is  made  to  increase  with  load,  thereby  making  the 
B.C.  voltage  rise  also. 

Recently  a  special  type  of  rotary  using  field  poles  in  two 
sections  has  been  developed;  in  this  rotary  the  ratio  of  E.M.F.'s 
is  not  fixed.  In  a  small  laboratory  machine  of  this  type,  by 
proper  adjustment  of  the  currents  in  the  two  sections  of  the  pole, 
the  B.C.  voltage  maybe  made  to  vary  from  90  to  135,  while  the 
impressed  A.C.  voltage  is  kept  constant  at  88.  This  is  a  very 
special  type  of  machine  and  not  much  used  as  yet. 

The  ratio  of  an  ordinary  rotary  having  the  A.C.  taps  separated 
by  the  angle  a,  measured  in  electrical  degrees,  is  given  by  the 
formula, 

B.C.  volts  a 

A.C.  volts  =  —     -7= X  sin  -. 

\/2  2 

There  are  several  methods  for  starting  a  rotary  and  bringing 
it  into  synchronism  with  the  A.C.  line.  Either  of  th3  two  methods 
described  for  a  synchronous  motor  may  be  used  and  there  is  in 


THE  SYNCHRONOUS  CONVERTER 


189 


the  rotary  the  additional  possibility  of  starting  from  the  D.C. 
end  as  a  D.C.  shunt  motor.  This  is  a  convenient  method  for 
use  in  the  laboratory  as  D.C.  power  is  always  available.  In 
substations  D.C.  power  may  not  be  available,  under  which  con- 
dition one  of  the  methods  employing  A.C.  power  must  be  used. 

For  starting  from  the  D.C.  end  it  is  necessary  to  have  a  D.C. 
line  of  at  least  the  rated  voltage  of  the  machine.  For  a  single 
phase  machine  rated  at  no  volts  A.C.  the  necessary  D.C.  voltage 
is  1107.707  =  157  volts;  for  a  three-phase  no  volt  A.C.  rotary, 
the  necessary  D.C.  voltage  is  iio/.6i2  =  i8i  volts.  A  D.C. 


220  Volt 
line 


FIG.  37 

starting  rheostat  is  used  as  with  a  D.C.  motor.  The  rheostat 
resistance  is  decreased  until  the  impressed  D.C.  voltage  is  just 
that  necessary  to  give  the  required  A.C.  voltage.  Then  the 
rotary  is  bought  into  synchronism  with  the  line  by  changing  the 
field  current;  a  synchroscope  or  synchronizing  lamps,  as  with 
the  synchronous  motor,  may  be  used  to  indicate  the  proper  time 
for  closing  the  synchronizing  switch. 

Two  runs  are  to  be  made  with  the  rotary,  one  running  from 
the  D.C.  end  to  determine  the  ratio  of  the  converter  and  its  pos- 
sible variation  and  the  second  to  get  the  operating  characteristics 
of  the  machine  when  running  from  the  A.C.  end,  as  it  is  normally 
designed  to  do. 

For  making  these  two  runs  it  is  convenient  to  make  connections 
as  given  in  Fig.  37.  The  rotary  represented  is  a  single-phase 
machine  designed  for  no  volts  on  the  A.C.  end. 


190  TESTING  OF  ELECTRICAL  MACHINERY 

In  testing  the  voltage  ratio  make  one  run,  keeping  the  impressed 
D.C.  voltage  constant,  and  vary  the  speed  through  as  wide  a 
range  as  possible  by  changing  the  field  strength;  read  A.C.  volts, 
D.C.  volts  and  speed.  Then  take  another  run  with  various  D.C. 
voltages  impressed,  keeping  the  speed  constant  by  variation  of 
field  strength ;  read  same  as  before. 

In  taking  the  load  run  the  machine  is  first  to  be  synchronized 
with  the  A.C.  line.  After  the  starting  rheostat,  R  has  been  so 
adjusted  that  the  rotary  A.C.  voltage  is  equal  to  that  of  the  A.C. 
line,  change  the  speed  (by  field  variations)  until  the  synchronizing 
lamps  indicate  synchronism,  and  then  close  the  synchronizing 
switch  in  the  middle  of  a  dark  period,  with  lamps  connected  as 
in  Fig.  37.  Of  course  "  bright  "  connections  of  lamps  may  be 
used  if  desired.  When  synchronized  the  D.C.  supply  line  is 
to  be  opened  and  the  switch  thrown  over  to  load  side. 

Adjust  the  value  of  field  strength  so  that  at  no  load  the  A.C. 
armature  current  is  a  minimum.  Leave  the  field  circuit  resistance 
constant  at  this  value  and  put  load  on  the  D.C.  end  of  the  rotary 
in  about  8  steps  between  zero  and  50  per  cent  overload.  Keep 
impressed  A.C.  voltage  constant.  Read  A.C.  volts,  amperes, 
and  watts,  field  current,  D.C.  load  current  and  D.C.  volts. 

On  one  curve  sheet  plot  the  ratio  values  obtained  in  first  run, 
using  ratio  'as  ordinates.  On  second  curve  sheet,  plot  curves  of 
D.C.  volts,  efficiency  and  power  factor,  using  D.C.  load  current 
as  abscissae  for  all  curves. 

QUESTIONS 

What  voltage  must  be  impressed  on  the  A.C.  end  of  a 
three-phase  converter  if  the  voltage  on  the  D.C.  end  is  to  be 
600? 

If  this  converter  is  to  be  compounded  50  volts  on  the  D.C. 
end,  how  much  must  the  voltage  impressed  on  the  A.C.  end 
increase  from  no  load  to  full  load  (neglecting  armature  drop)  ? 

Through  what  range  can  the  voltage  ratio  of  an  ordinary 
converter  be  varied? 


THE  SYNCHRONOUS  CONVERTER  191 

A  converter,  running  inverted  on  a  D.C.  line  of  no  volts, 
gives  what  voltage  on  its  slip  rings,  three-phase  machine  assumed? 

A  railway  converter  rated  at  1000  K.W.,  six  hundred  volts, 
would  draw  what  current  from  the  A.C.  line,  if  it  were  run 
single  phase  and  had  an  efficiency  of  93  per  cent? 

Explain  why  the  voltage  on  the  D.C.  end  of  a  converter, 
running  normally,  does  not  increase  as  the  field  strength  is 
increased. 


EXPERIMENT  VIII 

Parallel  Operation  of  Alternators;  Circulating  Current; 
Division  of  Load  Dependent  upon  Phase  Shifting.  At  present 
the  largest  sizes  in  which  it  is  feasible  to  build  A.C.  generators 
is  about  20,000  K.V.A.  To  equip  a  station  for  a  capacity  of 
100,000  K.V.A.  it  is  therefore  necessary  to  install  several  gene- 
rators, and  as  there  is  generally  only  one  set  of  bus  bars  and  one 


Loaa 


© 
B 


distribution  system,  it  is  necessary  to  so  connect  these  generators 
electrically,  that  they  all  supply  power  to  the  same  line.  The 
only  stable  connection  is  to  have  them  working  in  parallel,  and 
many  stations  have  as  many  as  ten  or  more  large  alternators  all 
connected  in  parallel  to  the  same  switchboard.  It  is  therefore 
important  to  investigate  the  operating  characteristics  of  such  a 
set  of  machines. 

It  will  first  be  shown  that  two  alternators  operating  in  series 
are  not  in  stable  equilibrium.  Two  single  phase  machines, 
connected  in  series,  supplying  a  load  drawing  a  current,  lagging 

192 


PARALLEL  OPERATION  OF  ALTERNATORS        193 

somewhat  behind  the  line  E.M.F.,  are  shown  in  Fig.  38.  Each 
machine  is  generating  the  same  voltage  and  it  is  supposed  that 
for  some  reason  that  machine  B  has  pulled  slightly  ahead  of 
machine  A  in  phase.  The  vector  diagram  of  E.M.Fs.  and  cur- 
rent is  given  in  Fig.  38.  The  vectors  OA  and  OB  represent 
the  machine  voltages,  OC  the  resultant  or  line  voltage,  and  OI 
the  line  current.  The  load  on  machine  B  is  equal  to  OB  X  OI 
Xcos  ft  and  that  on  machine  A  is  equal  to  OAxOIXcos  a. 
As  a  is  less  than  /?  it  is  evident  that  machine  B,  which  for  some 
reason  has  pulled  ahead  of  A  in  phase,  has  thereby  relieved 
itself  of  part  of  its  share  of  the  load.  This  effect  will  make  machine 


Load 


FIG.  39 

B  speed  up  still  more  because  any  ordinary  prime  mover  will 
increase  its  speed  if  load  is  taken  from  it.  B  will  continue  to 
get  ahead  of  A  in  phase  until  the  two  vectors  OA  and  OB  are 
practically  in  opposition,  under  which  condition  there  is  no  line 
voltage  and  therefore  no  load. 

The  polarities  marked  in  Fig.  38,  of  course,  are  true  only  at 
a  certain  instant,  but  the  two  machines  will  both  reverse  at  the 
same  time,  leaving  the  phase  of  E.M.Fs.  relative  to  one  another, 
the  same.  If,  however,  when  the  two  machines  have  pulled 
into  opposition,  with  respect  to  each  other,  the  load  circuit  is 
short  circuited  and  the  load  attached  as  in  Fig.  39,  the  conditions 
of  operation  and  load  distribution  are  stable.  The  vector  dia- 
gram given  in  Fig.  40  will  make  this  point  clear.  When  the  two 
machines  are  exactly  in  opposition,  with  respect  to  each  other, 


194  TESTING   OF   ELECTRICAL   MACHINERY 

the  E.M.F.  vectors  are  shown  at  OA  and  OB.  There  is  no 
resultant  E.M.F.  in  the  circuit  consisting  of  the  two  armatures 
and  connecting  bus  bars  and  therefore  there  is  no  current  flowing 
around  this  local  circuit.  The  two  machines  will,  under  these 
conditions,  divide  the  load  equally,  provided  that  the  two  arma- 
tures have  equal  impedances. 

In  Fig.  39  it  is  seen  that  although  the  two  E.M.Fs.  oppose 
one  another  around  the  local  armature  circuit,  they  act  together, 
in  parallel,  in  so  far  as  the  load  circuit  is  concerned. 

Suppose  now  that  machine  B  speeds  up  for  an  instant  with 
respect  to  A,  so  that  the  relative  phases  of  the  two  E.M.F.  vec- 
tors are  as  shown  as  OA  and  OB',  in  Fig.  40.  In  the  local  circuit 


•>A 


FIG.  40 

there  now  exists  a  resultant  voltage  OC  which  will  force  current 
to  flow  through  the  two  armatures,  in  addition  to  whatever  load 
current  the  two  machines  may  be  carrying.  As  the  inductance 
of  the  armature  is  much  greater  than  the  resistance,  this  local 
current  will  lag  nearly  90°  behind  the  E.M.F.  causing  it;  OI 
represents  this  current  in  phase,  behind  the  voltage  OC,  by  the 
angle  6  where 

armature  reactance 


tan  0  = 


armature  resistance 


Now  this  current  is  nearly  in  phase  with  the  voltageO£',and  hence 
is  a  load  on  machine  B,  while  it  is  a  motor  current  for  machine 
A,  as  it  is  nearly  180°  out  of  phase  with  OA.  The  real  effect 
of  this  current  is  therefore  to  take  some  of  the  load  from  machine 
A  and  put  it  on  machine  B\  this  will  tend  to  slow  down  B  and 
is  therefore  an  effect  which  tends  to  prevent  the  machines  leaving 
their  phase  position  of  180°  with  respect  to  one  another. 


PARALLEL   OPERATION   OF   ALTERNATORS  195 

The  previous  analysis  has  been  on  the  assumption  that  the 
two  machines  were  generating  the  same  voltage.  Suppose  now 
that  the  load  on  the  two  machines  is  equally  divided  (E.M.Fs., 
1 80°  apart)  and  the  excitation  of  machine  A  is  increased.  Will 
this  change  the  distribution  of  load?  By  reference  to  Fig.  41, 
it  is  seen  that  the  resultant  voltage  OC,  now  lies  in  phase  with 
OA.  The  resultant  local  current  01  is  shown  lagging  by  the 
angle  6  behind  OC;  this  current  OI  is  practically  a  wattless  cur- 
rent, as  it  is  in  90°  position,  nearly,  with  respect  to  both  E.M.Fs. 
As  it  is  not  in  phase  with  the  generator  E.M.Fs.,  it  cannot, 
with  non-inductive  load,  represent  load  current.  As  a  matter 
of  fact,  this  increase  in  voltage  of  machine  A  will  scarcely  affect 
the  load  distribution  at  all,  but  will  produce  a  current  which  flows 


in  the  local  circuit  only ;  it  is  practically  90°  out  of  phase  with  the 
E.M.Fs.  and  its  only  effect  is  to  tend  to  equalize  the  two  volt- 
ages OA  and  OB.  It  will  tend  to  magnetize  the  machine  B  and 
demagnetize  machine  A.  This  effect  of  armature  reaction  by 
the  circulating  current  will  change  the  voltage  of  the  line  some- 
what as  excitation  of  machine  A  is  varied. 

Referring  to  Fig.  40  it  is  evident  that  the  division  of  load 
depends  upon  the  angle  <*.  Now  the  only  way  of  changing  this 
angle  is  to  vary  the  driving  torque  of  the  prime  mover,  and  it  is 
in  this  fashion  that  load  is  distributed  between  the  different 
machines  in  a  station.  The  steam  supply  of  the  driving  engine  or 
turbine  is  generally  under  the  control  of  the  switchboard  operator. 

Summing  up  the  conclusions  reached  we  have:  the  division 
of  load  between  two  alternators  operating  in  parallel  depends 


196 


TESTING   OF   ELECTRICAL   MACHINERY 


only  upon  their  relative  phase  position;  the  load  division  cannot 
be  affected  by  varying  the  field  strength,  but  such  variation  of 
field  results  in  a  nearly  wattless  current  circulating  in  the  local 
circuit,  which  merely  heats  the  machines,  represents  no  power 
output  and  is  therefore  detrimental  to  the  operation  of  the 
machines. 

The  first  run  to  be  made  in  this  test  is  to  show  the  variation 
of  load  on  machine  B  by  variation  of  the  driving  torque,  and 
hence  the  phase  angle  a,  with  constant  excitation  of  5;  the 
second  is  run  to  keep  the  driving  torque  of  B  fixed,  and  to  vary 
the  excitation  of  B  both  above  and  below  normal  to  show  vari- 
ation of  circulating  current  and  independence  of  load  division. 

Load 


FIG.  42 

Make  connections  as  in  Fig.  42,  bring  machine  A  to  rated 
voltage  and  read  its  field  current.  Keep  it  constant  at  this 
value  throughout  the  test.  Put  upon  machine  A,  a  load  of 
about  one-half  its  rated  capacity.  Synchronize  machine  B  with 
machine  A  and  read  field  current,  watts,  output,  armature  cur- 
rent and  line  voltage.  Leave  the  field  current  fixed  at  this 
value  and  increase  the  torque  of  5's  prime  mover  in  such  steps 
as  will  produce  changes  in  the  armature  current  of  B  of  about 
one-quarter  rating.  Take  readings  up  to  50  per  cent  overload, 
reading  for  each  step  watts  and  armature  amperes  of  B  and 
line  voltage.  Get  the  values  of  a.  for  each  setting  of  load  by 
the  scheme  of  two  insulated  discs  used  in  experiment  No.  6. 

Then  reduce  the  torque  of  5's  prime  mover  until  the  wattmeter 
reads  zero.  Now  vary  the  field  current  of  B  in  such  steps  as 
produce  increments  in  the  armature  current  of  .about  one-quarter 


D.c. 


PARALLEL  OPERATION  OF  ALTERNATORS        197 

rating,  reading  for  each  setting,  field  current,  armature  current 
and  watts,  line  voltage  and  phase  position  of  £'s  armature. 

On  one  curve  sheet  plot  the  variations  of  watts  load  of  B  and 
phase  position  of  its  armature.  Calculate  also  the  value  of  the 
circulating  or  wattless  current  flowing  in  5's  armature  for  each 
reading.  Plot  load  and  circulating  current  against  phase  position 
as  abscissae.  On  a  second  sheet  plot  variations  of  load  in  watts, 
circulating  current  and  phase  position,  against  field  current  of 
B  as  abscissae. 

NOTE. — The  remarks  regarding  effect  on  load  distribution  of 
variation  of  5's  field  current  were  made  on  the  assumption  that 
the  angle  0  was  nearly  90°.  In  so  far  as  this  is  not  true  the 
conclusions  reached  are  more  or  less  inaccurate,  but  a  more 
detailed  discussion  makes  the  question  too  complex. 

QUESTIONS 

A  i2-pole  alternator  is  to  be  synchronized  with  a  6o-cycle 
line.  At  what  speed  must  it  be  run? 

Referring  to  Fig.  42,  suppose  the  readings  of  the  instruments 
on  machine  B  are  volts  no,  amperes  55,  watts  4850,  the  load 
circuit  being  non-inductive.  How  much  is  the  circulating  current 
between  the  two  machines?  If  machine  A  is  furnishing  to  the 
load  10  K.W.  of  power,  how  much  current  is  there  flowing  in  its 
armature? 

If  machine  A  is  generating  60  cycles  and,  before  synchronizing, 
the  lamps  flicker  twice  per  second,  how  fast  is  machine  B  running 
if  it  has  8  poles?  There  are  two  possible  answers  to  this  question. 

Why  should  the  field  currents  of  the  various  alternators  in 
a  station,  operating  in  parallel,  be  so  adjusted  that  the  circulating 
current  is  a  minimum? 

Suppose  that  the  maximum  safe  armature  current  of  machine 
B,  Fig.  42,  is  150  amperes,  and  also  that  the  fields  of  A  and  B 
are  so  adjusted  that  there  is  a  circulating  (wattless)  current  of 
45  amperes  flowing  between  them.  If  the  bus  bar  voltage  is  125, 
what  is  the  maximum  load  (in  K.W.)  that  machine  B  can  fur- 
nish to  the  load  circuit? 


EXPERIMENT  IX 


Current   and  Voltage  Relations  in   a  Three   Phase  Circuit; 
Measurement  of  Power  on  Non-inductive  and  Inductive  Loads. 

Practically  all  A.C.  power  is  generated,  transmitted  and  utilized 
by  polyphase  circuits  and  machines.  Of  all  polyphase  circuits 
the  three  phase  is  by  far  the  most  important. 


Line 


Li 


FIG.  43 

The  easiest  way  to  study  the  current  and  E,M.F.  relations 
in  such  a  circuit  is  by  first  considering  it  as  three  single  phase 
circuits.  The  problem  will  be  investigated  only  for  balanced 
loads,  i.e.,  a  polyphase  system  which  may  be  considered  as  made 
up  of  a  group  of  equally  loaded  single  phase  circuits. 

A  three  phase  load  may  be  connected  in  two  ways :  the  star  or 
Y  connection  shown  at  a,  Fig.  43,  and  the  mesh  or  A  shown  at  b. 
With  either  connection  only  three  wires  are  used.  It  is  apparent 
that  only  three  wires  are  needed  for  the  A  connection,  and  it 
may  be  shown  that  in  the  Y  connection  a  wire  connecting  to  the 

198 


THREE-PHASE    CURRENT   AND   VOLTAGE   RELATIONS       199 

center  or  neutral  point  of  the  load  is  unnecessary.  In  Fig.  44 
the  three  phases  are  supposed  separate,  each  phase  is  carrying 
the  same  magnitude  current  and  of  course  the  three  currents, 
represented  as  vectors,  are  120°  apart.  This  feature  of  the  three 
phase  system  is  a  result  of  the  method  of  placing  the  coils  on  the 
armature  of  the  three  phase  generator;  the  coils  are  placed  120° 
(electrical)  apart,  hence  generate  three  sine  E.M.Fs.  120°  apart, 
and  so  the  currents  from  such  a  generator  are  120°  apart. 


FIG.  44 

In  Fig.  44  the  three  single  phases  are  shown  at  a,  b,  and  c. 
The  three  lines,  i',  2'  and  3'  are  evidently  carrying  three  equal 
currents,  120°  apart  in  time.  If  then  these  three  lines  are  joined 
throughout  their  entire  length,  the  resultant  single  line  will  carry 
the  resultant  of  three  equal  sine  currents  spaced  120°  apart.  But 
such  resultant  is  zero,  and  therefore  the  combination  line  or 
neutral,  as  it  is  called,  is  useless  and  so  not  used.  The  three  single 
circuits,  joined  together  at  i',2/3'  then  constitute  a  Fload,  supplied 
with  three  phase  power  through  the  lines  i,  2,  3.  In  discussing 
voltage  and  current  relations  we  have, 

i  =  current  in  each  phase  of  load; 
e  =  voltage  across  one  phase; 
7= current  in  any  line; 
E  =  voltage  between  lines. 


200 


TESTING  OF  ELECTRICAL  MACHINERY 


In  the  Y  connection  of  load  it  is  evident  that  1=1.  To 
get  the  line  voltage  it  is  necessary  to  take  the  vector  differ- 
ence of  two  voltages,  each  of  magnitude  =  e,  spaced  120°  apart. 
The  voltage  between  lines  by  this  construction  gives  E  =  eV$.* 

In  the  delta  connection  E=e.  To  get  /  it  is  necessary  to  take  the 
vector  difference  of  two  currents  each  of  magnitude  i,  and  120° 
apart  in  phase,  and  this  gives  I=i\/?>. 

Now  no  matter  how  the  load  is  connected,  it  is  evident  that 
the  power  used  in  the  three  phases  is  equal  to  3^,  if  cos  $=i. 
Substituting  values  of  line  current  and  line  voltage  gives  power 
of  a  three-phase  load  =  £/V3,  and  this  holds  good  for  either  con- 


I- 


FIG.  45 


nection  of  load.  If  the  power  in  each  phase  is  equal  to  ei  cos  <£, 
then  in  terms  of  line  quantities  we  have,  watts  used  in  three- 
phase  load  =  ElV$  cos  <f>. 

To  measure  the  power  used  in  a  three  phase  line  it  is  not 
necessary  to  actually  measure  the  watts  in  each  phase  and  multiply 
by  three.  If  it  is  known  that  the  load  is  non-inductive,  then  the 
power  used  is  easily  determined  by  measuring  the  line  current 

*  The  derivations  of  the  formulae  for  voltage  and  current  relations,  as  well  as 
power  relations,  in  three-phase  circuits  are  not  introduced  here,  as  it  is  a  some- 
what more  involved  discussion  than  it  is  thought  well  to  incorporate  in  this  text. 
The  student  is  referred  to  Morecroft's  Laboratory  Manual  of  Alternating  Cur- 
rents, Experiment  XXV. 


THREE  PHASE    CURRENT   AND   VOLTAGE   RELATIONS      201 

and  voltage  and  multiplying  by  vj.  When  the  power  factor  is 
unknown,  as  is  generally  the  case,  another  method  must  be  used 
If  two  wattmeters  are  used  as  shown  in  Fig.  45  the  sum  of 
the  two  readings  will  always  be  the  power  used  in  the  three  phase 
circuit,  no  matter  what  the  power  factor  may  be  or  whether  the 
load  is  balanced  or  not  balanced.  If  the  power  factor  is  less 
than  .5,  one  wattmeter  will  indicate  negatively  and  then  the 
algebraic  sum  must  be  used,  not  the  arithmetical  sum.  For 
balanced  loads  it  may  be  shown  that  the  current  in  line  i  is  30° 
out  of  phase  with  the  voltage  between  1-2,  and  the  current  in 
line  3  is  30°  out  of  phase  with  the  voltage  between  lines  3-2. 
If  $  is  the  phase  angle  of  the  load,* 


from  which 


But  we  had  already  shown  that  this  is  the  power  used  in  the 
three  phase  load. 

A  convenient  switching  arrangement  for  making  measure- 
ments on  three  phase,  delta  loads  is  shown  in  Fig.  46.  Three 
single  pole,  double  throw  switches  are  so  inserted  that  the  amme- 
ters and  wattmeters  may  be  readily  transferred  from  the  line  to 
the  phase.  In  balancing  or  unbalancing  the  A  load  the  meters 
are  connected  in  the  phase  (switches  all  thrown  to  left  in  Fig.  46)  ; 
then  when  it  is  desired  to  read  line  values  the  switches  are  thrown 
to  the  right. 

With  connections  as  shown  in  Fig.  46,  with  meters  connected 
in  phases,  adjust  the  three  phases  so  that  the  load  is  balanced 
(using  non-inductive  load,  such  as  incandescent  lamps)  and  read 
the  three  phase  currents.  With  the  potential  coils  of  the  watt- 
meter connected  as  shown  in  Fig.  46  it  is  evident  that  when  the 
switches  are  thrown  to  the  right  the  wattmeters  will  read  the 
power  in  the  correspondingly  numbered  phases  ;  read  each  of  the 
meters. 

*See  note  bottom  of  page  200. 


202 


TESTING   OF   ELECTRICAL   MACHINERY 


Now,  leaving  the  load  fixed  at  this  value,  transfer  the  meters 
to  the  line,  by  throwing  the  switches  to  the  right ;  read  the  three 
ammeters.  The  three  meters  should  read  equal  to  each  other 
and  equal  to  phase  current  XA/^.  Connection  a  of  wattmeter 


Jj^j 


FIG.  46 

Ws  should  be  clipped  on  to  the  blade  of  switch  in  line  i  so  that 
this  switch  cannot  be  thrown  to  the  right  without  removing  the 
connection.  As  the  switches  are  all  thrown  to  the  right,  con- 
nection a  must  be  removed  from  the  blade  of  switch  i  and  con- 
nected to  the  blade  of  switch  2.  Wattmeters  W\  and  Ws  are 
now  connected  as  shown  in  Fig.  45  so  that  the  sum  of  their 
readings  should  give  the  total  three  phase  power,  that  is,  should 
check  with  the  sum  of  the  three  wattmeter  readings  in  the  pre- 
vious test.  In  getting  the  power  from  the  line  meters  the 
reading  of  W2  is,  of  course,  neglected. 

It  is  to  be  noted  that  any  two  of  the  wattmeters  connected 
in  the  lines  may  be  used  to  get  the  three  phase  power.  Thus, 
if  Wi  and  Wi  have  respectively  their  potential  coils  connected 
to  lines  i  and  3,  and  2  and  3,  the  sum  of  their  readings  will  also 
be  the  true  three  phase  power.  Carry  out  the  above  measure- 
ments for  two  values  of  current  with  balanced  load  and  for  two 
conditions  of  unbalanced  load. 

Next  close  the  switches  connecting  the  variable  inductances  in 
parallel  with  the  lamp  banks,  as  shown  in  Fig.  46.  The  lamps  in 
parallel  with  the  variable  inductance  make  it  possible  to  obtain  a 
lagging  load  of  adjustable  power  factor. 


THREE   PHASE   CURRENT   AND   VOLTAGE   RELATIONS       203 

With  switches  thrown  to  the  left  (meters  connected  in  phase) 
adjust  the  three  phases  for  equal  current  and  equal  power  factors. 
This  is  most  easily  done  by  unscrewing  all  the  lamps  in  their 
sockets  so  that  they  are  out  of  circuit,  leaving  only  the  three 
variable  inductances  connected  to  the  three  phase  line.  The 


0_j^wjnnr> 


inductances  are  then  adjusted  to  give  equal  currents  in  the  three 
phases.  Then  enough  lamps  are  connected  in  circuit  to  bring  the 
power  factor  up  to  the  desired  value,  connecting  the  same  number 
in  each  phase  to  keep  the  load  balanced.  The  power  factor  is  of 
course  determined  by  the  ratio  of  watts  per  phase  to  volt-amperes 
per  phase. 

With  inductive  load,  balanced,  take  a  set  of  phase  readings, 
and  line  readings  as  for  non-inductive  load.  Do  this  for  one 
adjustment  of  power  factor  about  0.8  and  one  about  0.4.  In 
the  latter  case  it  will  be  found  that  when  reading  watts  in  the  line, 
one  of  the  wattmeters  must  have  its  potential  leads  reversed 
to  get  a  reading  on  the  scale.  The  reading  of  this  meter  must 
be  reckoned  negative  in  obtaining  the  total  three  phase  power  from 
the  line  wattmeter  readings.  From  the  readings  obtained  from 
these  two  runs  check  the  formula  for  power  factor  in  a  three 


phase  circuit,   cos$ 


—      —  J2. 
L1  1/3 


In  this  formula, 


and  W2 


are  reading  of  line  wattmeters  while  E  and  /  are  average  values 
of  line  voltage  and  current.    The  value  of  cps<£  obtained  by 


204  TESTING   OF   ELECTRICAL   MACHINERY 

this  formula  should  check  with  the  value  obtained  from  the  ratio 
of  watts  per  phase  to  volt-amperes  per  phase,  which  values  have 
been  obtained. 

Take  another  set  of  readings  with  load  unbalanced  and  com- 
pare line  and  phase  values  as  before.  The  two  line  wattmeters 
will  still  give  the  total  three  phase  power  correctly,  but  the  for- 
mula for  power  factor  will  not  hold  (except  approximately)  for 
slightly  unbalanced  load  and  the  ratio  between  line  current 
and  phase  current  will  not  be  V$. 

The  same  sets  of  readings  are  to  be  now  taken  for  Y-connected 
load  as  have  been  obtained  for  A  loads.  The  connection  scheme 
of  Fig.  46  is  charged  as  indicated  in  Fig.  47;  the  three  cables 
which  formerly  connected  to  the  blades  of  the  switches  now  are 
connected  together  by  the  three  way  connector  a.  The  watt- 
meter potential  coils  being  connected  as  shown  each  wattmeter 
reads  the  power  used  in  its  respective  phase.  By  taking  the  two 
potential  leads  of  W\  and  Ws,  which  connect  to  the  neutral  point 
a  of  Fig.  47,  and  connecting  them  both  to  line  2,  W\  and  Ws  are 
properly  connected  for  reading  the  total  three  phase  power; 
the  sum  of  their  readings  should  check  with  the  sum  of  the  three 
wattmeter  readings  when  all  potential  coils  were  connected  at  a. 
Of  course  it  is  just  as  well  to  use  any  two  of  the  three  wattmeters; 
thus  if  the  potential  leads  of  W\  and  W%  (those  leads  connecting 
at  a  in  Fig.  47)  are  connected  to  line  3  then  their  sum  will  also 
give  the  total  three  phase  power. 

With  balanced  non-inductive  load  (inductances  disconnected) 
get  the  relation  between  phase  quantities  and  line  quantities. 
In  this  connection  evidently  the  phase  current  and  line  current 
are  equal,  and  it  is  the  line  voltage  and  phase  voltage  that  have 
the  V~3  relations.  The  sum  of  the  three  phase  wattmeter  readings 
will  check  with  the  two  line  wattmeter  readings  as  in  the  A-con- 
nected  load.  Take  other  readings  for  unbalanced  non-inductive 
load,  and  then  readings  for  inductive  load,  balanced  and  unbal- 
anced. For  balanced  loads  it  will  be  found  that  the  line  volt- 
age is  equal  to  the  phase  voltage  XV^,  and  that  the  power 
factor  formula  holds  good,  but  for  unbalanced  loads  neither 


THREE   PHASE   CURRENT   AND   VOLTAGE   RELATIONS       205 

of  these  relations  is  true.  However,  for  all  conditions  of  load 
the  sum  of  the  two  line  wattmeter  does  give  correctly  the  three 
phase  power. 

A  convenient  method  for  balancing  the  three  phase  F-con- 
nected  non-inductive  load  consists  in  opening  one  line  (say  line  3) 
and  then  adjusting  phases  i  and  2  for  equality  by  making  the 
voltage  drops  across  the  two  equal.  With  line  3  open  phases  i 
and  2  form  a  simple  series  connection  on  lines  i  and  2.  Hence, 
when  they  are  adjusted  for  equal  voltages  the  impedances  must 
be  the  same  as  the  current  is  the  same  in  both.  When  i  and  2 
have  been  adjusted  to  equality,  open  line  i  and  close  line  3.  Now 
adjust  phases  2  and  3  for  equal  voltage  drop  by  varying  phase 
3,  leaving  2  just  as  it  was  balanced  with  i.  When  3  is  adjusted 
equal  to  2  then  line  i  may  be  closed  and  the  load  will  be  balanced. 
In  the  case  of  the  inductive  load  the  inductances  should  first 
be  balanced,  with  no  lamps  connected,  then,  when  the  inductances 
are  equal  a  suitable  number  of  lamps  may  be  inserted  (inserting 
the  same  number  in  each  phase)  to  bring  the  power  factor  to 
the  desired  value. 

QUESTIONS 

What  are  the  advantages  of  three  phase  power  compared 
to  single  phase  power? 

An  alternator  generates  6600  volts  per  phase.  What  will 
be  its  rated  voltage  if  it  is  connected  in  A  and  in  F.  With  which 
connections  will  its  possible  power  output  be  the  greater? 

A  three  phase  induction  motor  has  an  efficiency  of  92  per  cent, 
and  is  delivering  75  horse-power  to  its  load.  Its  power  factor  is 
.83  and  line  voltage  is  440.  What  is  the  current  in  each  of  its 
supply  lines?  If  its  stator  winding  is  connected  in  A,  what  is  the 
current  in  each  phase  of  the  winding? 

A  60.000- volt  three  phase  transmission  line  is  delivering  10,000 
K.W.  of  power  to  an  induction  motor  load  of  power  factor  .80. 
What  is  the  current  in  each  wire  of  the  line?  If  three  trans- 
formers are  connected  to  the  line,  primaries  in  F,  and  secondaries 
in  A,  what  will  be  the  voltage  of  the  line  to  which  the  secondaries 
are  connected,  the  transformer  ratio  being  190  to  i? 


EXPERIMENT  X 
Single  Phase  Motors 

The  single  phase  induction  motor — The  repulsion-induction 
motor — The  single  phase  series  motor. — There  is  much  need  of  a 
satisfactory  single  phase  motor  in  small  sizes  (10  H.P.  or  less) 
because  the  power  delivered  to  the  small  consumer  is  generally 
single  phase;  for  such  service  any  one  of  the  above-mentioned 
types  is  available.  The  single  phase  series  motor  has  been  used 
in  a  few  cases  in  comparatively  large  sizes,  notably  the  installa- 
tion of  the  N.  Y.,  N.  H.  and  H.  Railroad. 

For  general  use  the  speed  of  the  single  phase  motor  should  be 
practically  independent  of  load;  for  such  work  the  single  phase 
series  motor  is  not  at  all  suited  and  one  of  the  other  two  types 
must  be  used.  On  the  other  hand,  when  the  motor  has  to  be 
used  on  both  continuous  and  alternating  current  power  the  series 
motor  is  the  only  one  which  will  function. 

As  was  pointed  out  in  Ex.  5  the  single  phase  induction  motor, 
as  such,  has  no  starting  torque  whatever,  but  if  it  is  brought  to 
nearly  synchronous  speed  by  some  means  or  other  its  action  is 
practically  the  same  as  that  of  a  polyphase  motor.  The  first 
motor  to  be  tested  is  of  this  type ;  it  starts  as  a  repulsion  motor, 
accelerates  as  such  until  operating  at  nearly  synchronous  speed 
when  a  centrifugal  device  throws  off  the  repulsion  motor  brushes 
and  clamps  a  short-circuiting  ring  against  the  commutator 
making  the  armature  the  equivalent  of  a  squirrel  cage  rotor,  the 
motor  then  operating  as  a  straight  single  phase  induction  motor. 

The  action  of  the  repulsion  motor  may  be  analyzed  by  refer- 
enece  to  Fig.  48.  The  two  field  coils  shown  at  A  and  B,  are  in 
series  and  connected  to  a  single  phase  supply;  on  the  armature 
we  imagine  a  short-circuited  turn,  shown  in  three  possible  posi- 
tions, i-i'j  2-2',  and  3-3'.  The  field  coils  produce  an  alternating 


SINGLE   PHASE   MOTORS 


207 


it 
si 


flux  which  threads  the  armature  so  that  the  short-circuited  turn 
will  have  currents  set  up  in  it  for  any  position  except  that  shown 
at  i-i';  in  this  position  the  plane  of  the  coil  is  parallel  to  the 

direction  of  the  magnetic  field 

so  there  is  no  E.M.F.  induced 
in  the  coil.  In  position  3-3' 
there  will  be  a  large  current 
in  the  short-circuited  turn  but 
no  torque  will  be  developed, 
the  plane  of  the  coil  being  per- 
pendicular to  the  direction  of 
the  flux.  At  some  intermediate 
position,  such  as  2-2',  current 
will  be  set  up  in  the  short- 
circuited  turn  and  torque  will 

be  developed  tending  to  make  the  coil  pull  into  position 
i-i'.  If  we  use  the  plane  of  coil  3-3' as  reference  the  torque 


1-1 


2-2 
Position  of  Coil  of  Fig.  48. 

FIG.  49 


and  current  in  the  short-circuited  turn  for  various  positions 
around  the  armature  are  about  as  shown  in  Fig.  49;  the 
direction  of  the  torque  reverses  as  the  coil  goes  through  posi- 
tion 3-3',  where  the  current  is  a  maximum. 


208  TESTING   OF   ELECTRICAL   MACHINERY 

In  the  actual  motor  the  armature  has  a  winding  just  like 
that  of  a  continuous  current  motor,  connected  to  a  commutator. 
Brushes,  180  electrical  degrees  apart,  make  contact  on  the 
commutator  and  these  brushes  are  short  circuited.  Such  a 
winding  (with  the  short-circuited  brushes)  is  nearly  equivalent 
to  one  short-circuited  turn,  the  plane  of  which  is  fixed  by  the 
position  of  the  brushes.  By  moving  the  brushes  around  the 
commutator  the  current  in  the  armature,  and  the  torque  devel- 
oped by  it,  vary  as  shown  in  Fig.  49.  As  the  armature  rotates 
under  the  influence  of  the  torque  developed,  the  equivalent 
short-circuited  turn,  to  which  we  have  supposed  the  actual 
winding  equivalent,  remains  in  the  same  angular  position,  this 
being  fixed  by  the  position  of  the  brushes,  which  remain  station- 
ary as  the  armature  rotates.  In  the  actual  motor  the  brush  posi- 
tion is  so  taken  that  the  angle  between  the  equivalent  coil  and 
position  3-3 '  (Fig.  48)  is  about  20  electrical  degrees. 

The  repulsion  motor  has  running  characteristics  like  those 
of  a  series  motor;  it  continually  speeds  up  unless  suitably  loaded. 
In  the  case  of  the  Wagner  motor  as  it  approaches  synchronous 
speed,  a  copper  ring,  carried  on  a  toggle  joint,  is  snapped  hard 
against  the  commutator,  short-circuiting  all  the  bars  of  the  com- 
mutator and  thus  making  the  armature  winding  equivalent  to 
the  squirrel  cage  rotor  winding  of  the  ordinary  small  induction 
motor.  Motors  built  to  operate  in  this  fashion  may  be  made 
to  develop  starting  torques  in  excess  of  the  rated,  full-load, 
torque. 

The  repulsion-induction  motor  has  a  commutator  and  arma- 
ture winding  similar  to  the  ordinary  continuous  current  motor 
and  has  two  pairs  of  brushes  (for  a  two-pole  motor)  nearly  90 
electrical  degrees  apart.  In  the  ordinary  form  of  this  type  of 
motor  one  pair  of  brushes  is  short  circuited  and  the  other  pair 
connects  to  two  taps  on  the  stator  winding,  as  indicated  in 
Fig.  50,  giving  what  is  called  the  compensated  repulsion-induc- 
tion motor.  The  electrical  actions  of  such  a  motor  are  too  com- 
plex to  analyze  in  a  brief  text  of  this  kind,  so  will  not  be  attempted. 
The  angle  between  the  brushes  A- A  and  brushes  B-B  is  properly 


SINGLE   PHASE   MOTORS 


209 


FIG.  50 


adjusted  at  the  factory  and  is  not  adjustable  thereafter.  By 
having  a  proper  angle  between  the  brushes  and  connecting 
brushes  B-B  to  the  proper 
points  on  the  stator  winding 
the  motor  gives  a  good  starting 
torque  and  has  a  speed-load 
curve  like  that  of  the  ordinary 
induction  motor,  with  the  dif- 
ference that  the  motor  may 
run  at  speeds  higher  than  syn- 
chronous speed.  The  power 
factor  of  the  motor  is  greatly 
affected  by  the  connection  of 
brushes  B-B  to  the  stator 
winding;  it  may  approximate 
unity  throughout  a  large  varia- 
tion of  load.  In  this  type  of  motor  all  brushes  remain  perma- 
nently on  the  commutator. 

The  single  phase  series  motor  is  electrically  similar  to  the 
continuous  current  series  motor;  there  are  certain  important 
changes  required  in  the  construction  due  to  the  fact  that  the 
field  flux  is  alternating  instead  of  constant.  All  of  the  magnetic 
circuit,  carrying  alternating  flux,  must  be  laminated,  thus 
requiring  laminated  poles  and  yoke;  due  to  the  hysteresis  and 
eddy  current  losses  in  the  field  iron  the  flux  density  in  the  whole 
field  structure  must  be  kept  much  lower  than  is  the  case  in  the 
continuous  current  motor.  In  order  to  obtain  a  reasonably  high- 
power  factor  in  this  type  of  motor  the  armature  ampere- turns 
must  be  neutralized  as  nearly  as  possible,  this  requiring  a  com- 
pensating winding  in  the  pole  faces.  The  compensating  winding 
is  connected  in  series  with  the  armature  and  main  field  windings. 

Due  to  a  transformer  effect  from  the  alternating  field  flux 
producing  heavy  currents  in  those  coils  short  circuited  by  the 
brushes  (which  coils  are  undergoing  commutation)  it  is  necessary 
to  use  so-called  "resistance  leads"  in  connecting  the  coil  junc- 
tions to  the  commutator  bars.  Instead  of  connecting  the  coil 


210  TESTING   OF   ELECTRICAL  MACHINERY 

junctions  directly  to  the  commutator  as  is  done  in  the  ordinary 
continuous  current  motor  this  connection  is  made  through  a 
piece  of  resistance  wire,  the  resistance  of  which  may  be  perhaps 
five  times  as  much  as  the  coil  resistance.  This  feature  of  con- 
struction practically  eliminates  sparking  at  the  commutator  in 
so  far  as  this  sparking  is  caused  by  the  effect  of  the  alternating 
field  flux.  Even  when  all  the  above  outlined  precautions  have 
been  taken  in  constructing  the  alternating  current  motor  its 
performance  on  an  alternating  current  line  is  much  inferior  to 
what  the  same  motor  will  give  when  operated  on  a  continuous 
current  line. 

The  single  phase  series  motor  is  practically  never  used  (except 
in  very  small  sizes,  such  as  required  by  portable  vcauum  cleaners) 
on  frequencies  higher  than  25  cycles;  due  to  difficulty  in  com- 
pensating the  armature  ampere-turns  the  output  and  power 
factor  fall  off  very  rapidly  as  the  frequency  is  increased  above 
that  for  which  the  motor  was  designed. 

With  the  single  phase  induction  motor  loaded  by  Prony 
brake  get  all  of  its  characteristics,  operating  as  an  induction 
motor  (speed,  current  input,  efficiency,  power  factor,  etc.); 
note  the  "pull-out"  torque.  Impressing  half  voltage  (to hold 
down  to  a  safe  limit  the  current  taken  by  the  motor)  get  the  char- 
acteristics of  the  motor  as  a  repulsion  motor  for  speeds  below 
that  at  which  the  centrifugal  device  throws  on  the  short-cir- 
cuiting ring.  With  half  voltage  impressed  and  the  rotor  clamped 
get  a  set  of  readings  similar  to  those  shown  in  Fig.  49,  reading 
torque  and  current  for  about  six  positions  on  either  side  of  the 
position  giving  zero  torque.  In  plotting  the  results  of  the  two 
half  voltage  runs  change  the  torque  to  what  it  would  have  been 
at  normal  voltage  by  multiplying  by  four,  and  the  current  to 
what  it  would  have  been  at  normal  voltage  by  multiplying  by 
two. 

With  the  compensated  induction-repulsion  motor  loaded  by 
brake  get  curves  of  speed,  input,  power  factor,  and  efficiency 
from  no  load  to  the  maximum  safely  obtainable  from  the 
motor. 


SINGLE    PHASE   MOTORS  211 

With  the  series  single  phase  motor  normally  connected  get  the 
ordinary  characteristics  operating  at  rated  voltage  and  fre- 
quency; if  loaded  by  Prony  brake  observe  proper  precautions 
to  prevent  the  motor  over-speeding  if  the  brake  should  acci- 
dentally come  off.  Get  the  same  characteristics  for  the  motor 
when  running  from  a  line  of  frequency  about  twice  that  for  which 
it  is  rated  (say  60  cycles  for  a  25-cycle  motor).  Get  the  same 
characteristics  with  the  motor  operating  from  a  continuous  cur- 
rent line,  of  voltage  equal  to  that  for  which  the  motor  is  rated; 
in  this  test  take  special  precautions  to  prevent  the  motor  from 
running  away. 

QUESTIONS 

How  does  the  starting  torque  of  a  repulsion  motor  vary  with 
the  impressed  voltage,  and  why? 

With  no  change  in  connections  would  a  6o-cycle  motor  of  the 
type  starting  as  repulsion  motor  operate  properly  on  a  25-cycle 
line?  Why? 

How  about  the  behavior  and  amount  of  power  output  safely 
available,  of  a  single  phase  10  H.P.  2 20- volt  6o-cycle  induction 
motor  is  run  from  a  no- volt  25-cycle  line? 

For  a  given  current  and  line  voltage  why  is  the  output  of  a 
series  motor  so  much  greater  on  a  continuous  current  line  than 
on  an  alternating  current  line? 

With  full  load  current  of  60  amperes  flowing  a  no- volt  25- 
cycle  motor  shows  a  power  factor  of  0.7.  Approximately  what 
current  will  it  draw  from  a  no- volt  6o-cycle  line  at  standstill? 


EXPERIMENT  XI 

The  Alternating  Current  Watt-hour  Meter.  Practically  all 
of  the  electric  power  sold  in  the  United  States  is  delivered  to  the 
customer  as  alternating  current  power,  hence  the  importance 
of  the  alternating  current  watt-hour  meter.  Although  the  com- 
mutator type  of  meter  as  well  as  the  mercury  motor  meter 
(explained  in  Ex.  14)  will  operate  on  alternating  current  lines 
the  induction  type  of  watt-hour  meter,  to  be  studied  in  this  test, 
is  so  far  superior  that  it  practically  monopolizes  the  field. 

An  elementary  sketch  of  the  essential  parts  of  the  induction 
meter  is  shown  in  Fig.  5 1 ,  by  reference  to  which  the  action  of  the 
meter  will  be  explained.  A  laminated  iron  frame,  of  the  form 
shown,  is  equipped  with  three-pole  pieces,  E,  D,  and  D' '.  The 
faces  of  these  three-pole  pieces  are  parallel  and  separated  by  suf- 
ficient distance  to  give  the  aluminum  disc,  A,  sufficient  mechan- 
ical clearance. 

The  upper  pole  piece,  E,  is  wound  with  two  coils,  B  and  C,  the 
former  (of  many  turns)  being  the  potential  coil  and  the  latter  of 
very  few  turns,  being  the  lag  coil.  The  two  lower  poles,  D  and 
D',  are  wound  in  series  with  each  other,  with  a  few  turns  of 
wire  of  sufficient  size  to  safely  carry  the  current  of  the  line  in 
which  the  meter  is  to  be  installed.  The  two  poles  are  wound 
in  opposite  directions  so  that  they  have  opposite  polarities. 

The  aluminum  disc,  A,  is  carried  on  the  steel  spindle  F,  the 
lower  bearing  of  which  is  carried  on  a  jewel.  On  the  upper  part 
of  the  spindle  is  a  worm  engaging  the  gear  train  which  records 
the  amount  of  energy  which  has  passed  the  meter.  The  disc  is 
caused  to  rotate  by  the  interaction  of  the  effects  of  the  potential 
and  current  coils.  The  disc  turns  between  the  poles  of  perma- 
nent magnets,  F  and  Ff,  the  eddy  currents  produced  by  the 
magnets  serving  to  limit  properly  the  speed  at  which  the  meter 
turns  for  a  definite  load. 

212 


THE  ALTERNATING  CURRENT  WATT-HOUR  METER 


213 


The  coil  B  is  made  up  of  sufficient  number  of  turns  that  its 
reactance  is  sufficient  to  permit  the  coil  being  connected  directly 
across  the  line  in  which  the  meter  is  connected,  this  being  gen- 
erally no  volts.  The  current  in  coil  B  will  lag  nearly  90  degrees 
behind  the  voltage  impressed  on  its  terminals  and  the  flux  in 


Laminated 
Iron  Frame 


FIG.  51 


the  potential  pole  will  be  in  phase  with  this  current  (neglecting 
for  the  moment  the  effect  of  the  lag  coil) .  Of  course,  it  is  impos- 
sible to  make  the  current  lag  as  much  as  90  degrees  behind  the 
E.M.F.  because  the  coil  must  have  some  resistance;  it  probably 
lags  about  80  degrees  in  the  average  meter.  To  make  the  meter 
operate  properly,  however,  it  is  necessary  that  the  flux  which 
passes  from  the  potential  pole  into  the  disc,  A,  be  exactly  90 
degrees  behind  the  phase  of  the  E.M.F.  impressed  on  the  meter. 
This  is  the  function  of  the  lag  coil,  which  is  generally  short-cir- 
cuited through  a  suitable  resistance. 

To  show  the  effect  of  the  lag  coil  we  refer  to  Fig.  52.  The 
line  voltage,  impressed  on  coil  B,  is  shown  at  OE\  the  ampere- 
turns  (M.M.F.)  produced  by  the  current  flowing  in  the  coil  are 
shown  at  OB,  about  80  degrees  behind  the  voltage.  This  M.M.F. 
will  produce  a  flux  in  phase  with  itself  which,  by  its  rate  of  change, 
generates  a  voltage  in  coil  C,  which  voltage  will  be  90  degrees 
behind  the  flux;  it  is  shown  at  OC.  This  voltage  will  cause  a 


214 


TESTING   OF   ELECTRICAL  MACHINERY 


FIG.  52 


current  OE  to  flow  in  the  coil  C  (as  it  is  short-circuited)  and  this 
current  will  lag  behind  the  voltage  OC  by  an  amount  controlled 

by  the  amount  of  resist- 
ance in  the  wire  used  to 
short-circuit  coilC.  The 
M.M.F.  due  to  the  cur- 
rent in  C  is  in  phase  with 

— >       the  current  OE   and  is 
6 

given  by  the  vector  OF. 
The  M.M.F.  producing 
flux  through  the  pole  E 
is  then  the  vector  result- 
ant of  the  M.M.Fs.  of 
the  two  coils;  in  Fig.  52 
it  is  shown  at  OD,  just 
90°  behind  the  E.M.F. 

impressed  on  coil  B.     The  flux  from  pole  E  is  therefore  properly 
shown  at  OG. 

A  plan  of  the  disc  is  given  in  Fig.  53,  it  shows  the  relative 
positions  of  the  three  poles  and  the  retarding  magnets  F,  Ff. 
The  alternating  flux  from  the  current  poles  induces  in  the  disc 
eddy  currents  which  flow  in  the  disc  about  as  indicated  by  the 
lines  of  Fig.  53 ;  due  to  the  opposite  polarities  of  the  two  current 
poles  the  eddy  currents  around  the  poles  will  be  in  opposite 
directions  as  shown.  It  will  be  noticed,  however,  that  both  cur- 
rents flow  in  the  same  direction  under  the  potential  pole.  These 
eddy  currents  in  the  disc  will  be  practically  in  phase  with  the 
E.M.F.  inducing  them;  this  E.M.F.,  which  is  caused  by  the  rate 
of  change  of  flux  through  the  current  poles  will  be  90°  behind 
the  current  exciting  the  current  poles,  that  is,  90°  behind  the 
load  current.  If  the  load  connected  to  the  meter  is  non-inductive 
the  load  current  will  be  in  phase  with  the  line  E.M.F.;  we  have 
previously  shown  that  the  flux  into  the  disc  from  the  potential 
pole  is  90°  behind  the  line  E.M.F.  so  that  it  is  now  evident  that 
the  eddy  currents  shown  in  Fig.  53  will  be  in  phase  with  the  flux 
from  the  potential  pole.  The  disc  will  therefore  experience  a 


THE   ALTERNATING   CURRENT   WATT-HOUR   METER       215 

torque  tending  to  turn  it,  this  torque  being  proportional  to  the 
flux  from  the  potential  pole  and  to  the  strength  of  the  eddy  cur- 
rents around  the  current  coils,  which  in  turn,  are  proportional 
to  the  load  current. 

One-quarter  of  a  cycle  after  the  time  assumed  in  the  above 
analysis  the  currents  around  the  current  poles  will  be  zero,  and 
the  flux  from  the  current  poles  will  be  a  maximum.  The  flux 
from  the  potential  pole  will  be  zero  at  this  time  but  there  will  be 


Current 
Poles 


Potential 
Pole 


FIG.  53 


eddy  currents  in  the  disc  produced  by  the  changing  flux  from  the 
potential  pole.  The  reaction  between  these  eddy  currents 
and  the  flux  from  the  current  poles  will  again  give  a  torque  to 
the  disc,  tending  to  turn  it  in  the  same  direction  as  does  the 
torque  previously  analyzed.  The  disc  therefore  experiences 
a  torque,  the  average  value  of  which  depends  upon  the  product 
of  the  line  current  and  line  E.M.F.,  that  is,  to  the  power  flowing 
through  the  meter. 

It  will  be  seen  that  the  average  value  of  this  torque  will  be 
a  maximum  (for  a  certain  voltage  and  current),  when  the  current 
through  the  meter  and  the  voltage  on  the  potential  coil  are  in 
phase  with  each  other;  as  the  phase  between  the  two  increases  the 
average  torque  will  diminish  until,  with  a  phase  difference  of 
90°  between  current  and  voltage,  the  average  torque  is  zero. 


216 


TESTING   OF   ELECTRICAL   MACHINERY 


Analysis  of  the  action  of  the  meter,  as  well  as  actual  test,  shows 
that  the  average  torque  is  proprtional  to  the  cosine  of  the  angle 
between  current  and  voltage,  that  is,  to  the  power  factor  of  the 
load. 

The  disc  will  speed  up  until  the  driving  torque  is  just  bal- 
anced by  the  eddy  current  drag  on  magnets  F,  F'  which  drag  is 
directly  proportional  to  the  speed;  hence  the  disc  will  rotate 
at  a  speed  fixed  by  the  power  (El  cos  </>)  being  supplied  to  the 
load. 

Just  as  in  the  case  of  the  direct-current  watt-hour  meter 
some  special  attachment  must  be  used  to  make  the  meter  indicate 


FIG.  54 

accurately  at  light  loads;  the  starting  friction  of  the  ordinary 
watt-hour  meter  is  such  that  there  is  required  a  considerable 
percentage  of  the  full-load  current  to  overcome  it  and  hence  at 
light  loads  the  meter  would  not  run  at  all.  To  neutralize  this 
starting  friction  there  is  attached  to  the  potential  pole  a  small 
adjustable  copper  plate,  so  mounted  that  it  can  be  moved  parallel 
to  the  face  of  the  potential  pole;  it  is  attached  so  that  it  can 
be  swung  to  cover  more  or  less  of  the  face  of  the  potential  pole. 
This  arrangement  is  called  the  starting  plate,  sometimes  the 
shading  plate  or  shading  coil.  The  action  of  this  plate  may 
be  explained  with  the  help  of  Fig.  54,  which  shows  a  plan  of  the 
face  of  the  potential  pole  and  the  starting  plate.  This  plate  is 
pivoted  at  some  point  (o)  so  that  it  can  be  swung  under  the  pole. 


THE    ALTERNATING    CURRENT   WATT-HOUR   METER        217 

Flux  from  the  potential  pole  will  induce  currents  in  this  plate; 
these  currents  in  the  starting  plate  will  induce  currents  in  the 
disc  A,  which,  reacting  with  the  flux  from  the  unshaded  portion 
of  the  potential  pole,  will  produce  a  slight  turning  effort.  The 
amount  of  the  turning  effort  is  controlled  by  the  position  of  the 
starting  plate. 

An  induction  watt-meter  may  be  single  phase,  two  or  three 
wire,  or  it  may  be  polyphase;  in  either  of  the  two  latter  cases  it 
consists  of  two  single-phase  meters  on  the  same  shaft.  These 
two  element  meters  are  generally  tested  by  connecting  the  two 
sets  of  current  coils  in  series  and  the  potential  coils  in  parallel 
and  then  loading  the  meter  single  phase. 

In  adjusting  the  meter  to  make  it  run  accurately  one  run  is 
made  at  light  loads  (5-10  per  cent  of  rating)  and  the  starting 
plate  varied  in  position  until  the  required  accuracy  is  obtained. 
At  full  load  the  meter  speed  is  adjusted  by  moving  the  permanent 
magnets  in  or  out  from  the  center  of  the  rotating  disc,  in  to 
increase  the  speed,  and  vice  versa. 

The  accuracy  to  which  the  meter  should  be  adjusted  depends 
upon  the  ruling  of  the  local  authorities ;  in  the  case  of  New  York 
the  meter  must  be  within  1.5  per  cent  of  correct  indication  at 
full  load  and  at  5  per  cent  of  full  load  the  allowed  deviation  is  plus 
or  minus  3  per  cent  from  accurate  indication.  The  above 
figures  are  for  a  load  of  100  per  cent  power  factor;  for  75  per  cent 
and  50  per  cent  power  factor  the  accuracy  at  full  load  must  be 
within  2  per  cent  and  4  per  cent  respectively. 

The  induction  meter  may  be  tested  in  the  same  fashion  as 
that  described  for  the  continuous  current  meter,  in  Ex.  14, 
but  the  uniform  practice  nowadays  is  to  use  a  portable  rotating 
induction  meter  as  a  secondary  standard,  this  being  frequently 
checked  with  some  other  standard  rotating  instrument  which  is 
not  carried  around.  The  potential  circuits  of  both  meters  should 
be  connected  to  the  power  line  at  the  same  place,  between  the 
power  supply  and  the  first  of  the  two  meters,  so  that  neither 
meter  is  affected  by  the  power  consumed  in  the  two  potential 
circuits.  The  two  current  coils  are  connected  in  series  and  in 


218 


TESTING   OF   ELECTRICAL   MACHINERY 


series  with  the  load.  In  case  the  load  on  the  meter  is  measured 
by  indicating  instruments  the  potential  coil  of  the  watt-hour 
meter  as  well  as  that  of  the  indicating  watt-meter  and  the  volt- 
meter should  all  be  connected  at  the  same  place,  between  the 
power  supply  and  the  first  of  the  meters;  this  is  indicated  in 
Fig-  55-  Unless  this  precaution  is  taken  the  test  may  be  in  error 
by  an  amount  depending  on  the  power  consumption  in  the 
various  potential  circuits.  With  the  connection  scheme  shown 
in  Fig.  55  the  meters  record  the  power  used  in  the  load  plus  that 


FIG.  55 


used  in  the  three-current  coils ;  the  amount  used  in  the  potential 
circuits  is  not  recorded. 

The  meter  is  to  be  tested,  as  found,  for  loads  of  5  per  cent, 
10  per  cent,  50  per  cent,  100  per  cent,  and  150  per  cent  load,  at 
unity  power  factor  and  at  0.7  power  factor,  lagging  current.  It  is 
then  to  be  adjusted  to  the  required  accuracy  at  5  per  cent  and 
100  per  cent  load,  unity  power  factor,  the  light  load  adjustment 
being  made  by  the  position  of  the  starting  plate  and  the  full- 
load  adjustment  by  the  position  of  the  magnets.  When  these 
adjustments  have  been  made  take  two  more  runs  similar  to 
those  first  made  to  see  how  the  calibration  holds  throughout  the 
range  of  the  meter,  at  the  two  power  factors. 

In  case  a  rotating  watt-hour  meter  is  used  for  the  calibration 
it  is  connected  in  series  with  the  meter  to  be  tested  and  the  load 
is  adjusted  to  that  at  which  the  test  is  to  be  made.  The  potential 
circuit  of  the  rotating  standard  is  opened  or  closed  by  a  push- 
button switch;  by  closing  this  switch  the  rotating  meter  is  made 


THE    ALTERNATING   CURRENT   WATT-HOUR   METER       219 

to  run,  the  meter  being  "dead"  until  the  potential  circuit  is 
closed  even  though  there  may  be  full-load  current  flowing  through 
the  current  coil.  The  reading  of  the  standard  meter  is  noted. 
By  means  of  a  stop  watch  (an  ordinary  watch  may  be  used  if  no 
stop  watch  is  available)  the  time  is  taken  for  a  suitable  whole 
number  of  revolutions  of  the  test  meter,  such  a  number  of  revolu- 
tions being  taken  that  the  time  required  is  between  one  and  two 
minutes.  During  this  same  interval  of  time  the  standard  meter 
must  be  recording:  this  is  most  conveniently  done  by  the  tester 
having  the  potential  circuit  switch  of  the  standard  meter  in  one 
hand  and  the  stop  watch  in  the  other ;  as  the  mark  on  the  disc 
of  the  test  meter  passes  a  convenient  point  (such  as  under  the 
edge  of  a  pole)  the  stop  watch  and  standard  meter  are  both 
started;  when  the  test  meter  has  made  the  required  number  of 
revolutions  they  are  both  stopped.  The  calibration  constant  of 
both  the  standard  and  test  meter  being  known  the  true  watts 
and  the  watts  indicated  by  the  test  meter  are  obtained  and  thus 
the  accuracy  of  the  test  meter  determined. 

In  case  indicating  meters  are  used  to  check  the  test  meter 
the  load  is  adjusted  to  the  required  amount  and  power  factor  by 
readings  of  voltmeter,  ammeter,  and  wattmeter;  the  time  for  a 
convenient  number  of  revolutions  of  the  test  meter  is  taken  as 
before,  the  load  being  obtained  by  taking  the  average  of  the 
indicating  wattmeter  reading  during  the  minute  or  so  the  test 
is  being  run. 

In  the  above  runs  the  meter  constant  must  be  known;  this  is 
different  for  the  various  meters  on  the  market  but  can  be  readily 
obtained  as  outlined  in  Ex.  14. 

For  each  setting  of  load,  for  which  the  meter  is  to  be  tested, 
three  sets  of  readings  should  be  taken,  these  not  to  be  regarded 
as  accurate  if  they  differ  from  the  average  by  more  than  i  per 
cent.  If  they  do,  other  runs  should  be  made  until  three  do 
agree  with  their  average  to  within  i  per  cent;  the  average  of 
these  three  readings  is  to  be  taken  as  the  meter  reading. 

Draw  curves  of  meter  calibration  as  found,  and  as  left,  for 
the  two  different  power  factors,  plotting  as  abscissae  per  cent 


220  TESTING   OF   ELECTRICAL  MACHINERY 

of  full  load  and  as  ordinates  the  ratio  of  meter  watts   to  true 
watts. 

QUESTIONS 

How  does  the  torque  due  to  the  starting  plate  vary  with 
speed?  How  about  that  due  to  bearing  friction? 

Why  does  the  magnet  adjustment,  carried  out  for  full  load, 
have  negligible  effect  on  the  light  load  adjustment? 

Considering  separately  the  potential  coil,  current  coils,  and 
damping  disc,  what  will  be  the  effect  of  increasing  temperature 
on  the  accuracy  of  the  meter? 

For  the  same  diameter  and  weight  of  disc,  and  same  braking 
effort,  which  requires  stronger  magnets,  an  aluminum  disc  or 
one  of  copper? 

What  would  be  the  effect  on  the  speed  of  the  meter  of  putting 
a  thin  sheet  of  copper  over  the  poles  of  the  current  coils,  between 
the  disc  and  the  pole  faces? 

With  a  given  disc  how  does  the  braking  effort  vary  with  the 
magnet  strength,  and  why? 

How  about  connecting  a  no- volt  6o-cycle  induction  meter  to 
a  no- volt  25-cycle  line? 


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